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+INTRODUCTION
+There is abundant evidence of a widened and deepened interest in modern
+science. How could it be otherwise when we think of the magnitude and
+the eventfulness of recent advances?
+But the interest of the general public would be even greater than it is
+if the makers of new knowledge were more willing to expound their
+discoveries in ways that could be "understanded of the people." No one
+objects very much to technicalities in a game or on board a yacht, and
+they are clearly necessary for terse and precise scientific description.
+It is certain, however, that they can be reduced to a minimum without
+sacrificing accuracy, when the object in view is to explain "the gist of
+the matter." So this OUTLINE OF SCIENCE is meant for the general reader,
+ who lacks both time and opportunity for special study, and yet would
+ take an intelligent interest in the progress of science which is making
+ the world always new.
+ The story of the triumphs of modern science is one of which Man may well
+ be proud. Science reads the secret of the distant star and anatomises
+ the atom; foretells the date of the comet's return and predicts the
+ kinds of chickens that will hatch from a dozen eggs; discovers the laws
+ of the wind that bloweth where it listeth and reduces to order the
+ disorder of disease. Science is always setting forth on Columbus
+ voyages, discovering new worlds and conquering them by understanding.
+ For Knowledge means Foresight and Foresight means Power.
+ The idea of Evolution has influenced all the sciences, forcing us to
+ think of _everything_ as with a history behind it, for we have travelled
+ far since Darwin's day. The solar system, the earth, the mountain
+ ranges, and the great deeps, the rocks and crystals, the plants and
+ animals, man himself and his social institutions--all must be seen as
+ the outcome of a long process of Becoming. There are some eighty-odd
+ chemical elements on the earth to-day, and it is now much more than a
+ suggestion that these are the outcome of an inorganic evolution, element
+ giving rise to element, going back and back to some primeval stuff, from
+ which they were all originally derived, infinitely long ago. No idea has
+ been so powerful a tool in the fashioning of New Knowledge as this
+ simple but profound idea of Evolution, that the present is the child of
+ the past and the parent of the future. And with the picture of a
+ continuity of evolution from nebula to social systems comes a promise of
+ an increasing control--a promise that Man will become not only a more
+ accurate student, but a more complete master of his world.
+ It is characteristic of modern science that the whole world is seen to
+ be more vital than before. Everywhere there has been a passage from the
+ static to the dynamic. Thus the new revelations of the constitution of
+ matter, which we owe to the discoveries of men like Professor Sir J. J.
+ Thomson, Professor Sir Ernest Rutherford, and Professor Frederick Soddy,
+ have shown the very dust to have a complexity and an activity heretofore
+ unimagined. Such phrases as "dead" matter and "inert" matter have gone
+ by the board.
+ The new theory of the atom amounts almost to a new conception of the
+ universe. It bids fair to reveal to us many of nature's hidden secrets.
+ The atom is no longer the indivisible particle of matter it was once
+ understood to be. We know now that there is an atom within the
+ atom--that what we thought was elementary can be dissociated and broken
+ up. The present-day theories of the atom and the constitution of matter
+ are the outcome of the comparatively recent discovery of such things as
+ radium, the X-rays, and the wonderful revelations of such instruments as
+ the spectroscope and other highly perfected scientific instruments.
+ The advent of the electron theory has thrown a flood of light on what
+ before was hidden or only dimly guessed at. It has given us a new
+ conception of the framework of the universe. We are beginning to know
+ and realise of what matter is made and what electric phenomena mean. We
+ can glimpse the vast stores of energy locked up in matter. The new
+ knowledge has much to tell us about the origin and phenomena, not only
+ of our own planet, but other planets, of the stars, and the sun. New
+ light is thrown on the source of the sun's heat; we can make more than
+ guesses as to its probable age. The great question to-day is: is there
+ _one_ primordial substance from which all the varying forms of matter
+ have been evolved?
+ But the discovery of electrons is only one of the revolutionary changes
+ which give modern science an entrancing interest.
+ As in chemistry and physics, so in the science of living creatures there
+ have been recent advances that have changed the whole prospect. A good
+ instance is afforded by the discovery of the "hormones," or chemical
+ messengers, which are produced by ductless glands, such as the thyroid,
+ the supra-renal, and the pituitary, and are distributed throughout the
+ body by the blood. The work of physiologists like Professor Starling and
+ Professor Bayliss has shown that these chemical messengers regulate what
+ may be called the "pace" of the body, and bring about that regulated
+ harmony and smoothness of working which we know as health. It is not too
+ much to say that the discovery of hormones has changed the whole of
+ physiology. Our knowledge of the human body far surpasses that of the
+ past generation.
+ The persistent patience of microscopists and technical improvements like
+ the "ultramicroscope" have greatly increased our knowledge of the
+ invisible world of life. To the bacteria of a past generation have been
+ added a multitude of microscopic _animal_ microbes, such as that which
+ causes Sleeping Sickness. The life-histories and the weird ways of many
+ important parasites have been unravelled; and here again knowledge means
+ mastery. To a degree which has almost surpassed expectations there has
+ been a revelation of the intricacy of the stones and mortar of the house
+ of life, and the microscopic study of germ-cells has wonderfully
+ supplemented the epoch-making experimental study of heredity which began
+ with Mendel. It goes without saying that no one can call himself
+ educated who does not understand the central and simple ideas of
+ Mendelism and other new departures in biology.
+ The procession of life through the ages and the factors in the sublime
+ movement; the peopling of the earth by plants and animals and the
+ linking of life to life in subtle inter-relations, such as those between
+ flowers and their insect-visitors; the life-histories of individual
+ types and the extraordinary results of the new inquiry called
+ "experimental embryology"--these also are among the subjects with which
+ this OUTLINE will deal.
+ The behaviour of animals is another fascinating study, leading to a
+ provisional picture of the dawn of mind. Indeed, no branch of science
+ surpasses in interest that which deals with the ways and habits--the
+ truly wonderful devices, adaptations, and instincts--of insects, birds,
+ and mammals. We no longer deny a degree of intelligence to some members
+ of the animal world--even the line between intelligence and reason is
+ sometimes difficult to find.
+ Fresh contacts between physiology and the study of man's mental life;
+ precise studies of the ways of children and wild peoples; and new
+ methods like those of the psycho-analyst must also receive the attention
+ they deserve, for they are giving us a "New Psychology" and the claims
+ of psychical research must also be recognised by the open-minded.
+ The general aim of the OUTLINE is to give the reader a clear and concise
+ view of the essentials of present-day science, so that he may follow
+ with intelligence the modern advance and share appreciatively in man's
+ continued conquest of his kingdom.
+ J. ARTHUR THOMSON.
+ I
+ THE ROMANCE OF THE HEAVENS
+ THE SCALE OF THE UNIVERSE--THE SOLAR SYSTEM
+ The story of the triumphs of modern science naturally opens with
+ Astronomy. The picture of the Universe which the astronomer offers to us
+ is imperfect; the lines he traces are often faint and uncertain. There
+ are many problems which have been solved, there are just as many about
+ which there is doubt, and notwithstanding our great increase in
+ knowledge, there remain just as many which are entirely unsolved.
+ The problem of the structure and duration of the universe [said the
+ great astronomer Simon Newcomb] is the most far-reaching with which
+ the mind has to deal. Its solution may be regarded as the ultimate
+ object of stellar astronomy, the possibility of reaching which has
+ occupied the minds of thinkers since the beginning of civilisation.
+ Before our time the problem could be considered only from the
+ imaginative or the speculative point of view. Although we can to-day
+ attack it to a limited extent by scientific methods, it must be
+ admitted that we have scarcely taken more than the first step toward
+ the actual solution.... What is the duration of the universe in
+ time? Is it fitted to last for ever in its present form, or does it
+ contain within itself the seeds of dissolution? Must it, in the
+ course of time, in we know not how many millions of ages, be
+ transformed into something very different from what it now is? This
+ question is intimately associated with the question whether the
+ stars form a system. If they do, we may suppose that system to be
+ permanent in its general features; if not, we must look further for
+ our conclusions.
+ The Heavenly Bodies
+ The heavenly bodies fall into two very distinct classes so far as their
+ relation to our Earth is concerned; the one class, a very small one,
+ comprises a sort of colony of which the Earth is a member. These bodies
+ are called _planets_, or wanderers. There are eight of them, including
+ the Earth, and they all circle round the sun. Their names, in the order
+ of their distance from the sun, are Mercury, Venus, Earth, Mars,
+ Jupiter, Saturn, Uranus, Neptune, and of these Mercury, the nearest to
+ the sun, is rarely seen by the naked eye. Uranus is practically
+ invisible, and Neptune quite so. These eight planets, together with the
+ sun, constitute, as we have said, a sort of little colony; this colony
+ is called the Solar System.
+ The second class of heavenly bodies are those which lie _outside_ the
+ solar system. Every one of those glittering points we see on a starlit
+ night is at an immensely greater distance from us than is any member of
+ the Solar System. Yet the members of this little colony of ours, judged
+ by terrestrial standards, are at enormous distances from one another. If
+ a shell were shot in a straight line from one side of Neptune's orbit to
+ the other it would take five hundred years to complete its journey. Yet
+ this distance, the greatest in the Solar System as now known (excepting
+ the far swing of some of the comets), is insignificant compared to the
+ distances of the stars. One of the nearest stars to the earth that we
+ know of is Alpha Centauri, estimated to be some twenty-five million
+ millions of miles away. Sirius, the brightest star in the firmament, is
+ double this distance from the earth.
+ We must imagine the colony of planets to which we belong as a compact
+ little family swimming in an immense void. At distances which would take
+ our shell, not hundreds, but millions of years to traverse, we reach
+ the stars--or rather, a star, for the distances between stars are as
+ great as the distance between the nearest of them and our Sun. The
+ Earth, the planet on which we live, is a mighty globe bounded by a crust
+ of rock many miles in thickness; the great volumes of water which we
+ call our oceans lie in the deeper hollows of the crust. Above the
+ surface an ocean of invisible gas, the atmosphere, rises to a height of
+ about three hundred miles, getting thinner and thinner as it ascends.
+ [Illustration: LAPLACE
+ One of the greatest mathematical astronomers of all time and the
+ originator of the nebular theory.]
+ [Illustration: _Photo: Royal Astronomical Society._
+ PROFESSOR J. C. ADAMS
+ who, anticipating the great French mathematician, Le Verrier, discovered
+ the planet Neptune by calculations based on the irregularities of the
+ orbit of Uranus. One of the most dramatic discoveries in the history of
+ Science.]
+ [Illustration: _Photo: Elliott & Fry, Ltd._
+ PROFESSOR EDDINGTON
+ Professor of Astronomy at Cambridge. The most famous of the English
+ disciples of Einstein.]
+ [Illustration: FIG. 1.--DIAGRAMS OF THE SOLAR SYSTEM
+ THE COMPARATIVE DISTANCES OF THE PLANETS
+(Drawn approximately to scale)
+ The isolation of the Solar System is very great. On the above scale the
+ _nearest_ star (at a distance of 25 trillions of miles) would be over
+ _one half mile_ away. The hours, days, and years are the measures of
+ time as we use them; that is: Jupiter's "Day" (one rotation of the
+ planet) is made in ten of _our hours_; Mercury's "Year" (one revolution
+ of the planet around the Sun) is eighty-eight of _our days_. Mercury's
+ "Day" and "Year" are the same. This planet turns always the same side to
+ the Sun.]
+ [Illustration: THE COMPARATIVE SIZES OF THE SUN AND THE PLANETS (Drawn
+ approximately to scale)
+ On this scale the Sun would be 17-1/2 inches in diameter; it is far
+ greater than all the planets put together. Jupiter, in turn, is greater
+ than all the other planets put together.]
+ Except when the winds rise to a high speed, we seem to live in a very
+ tranquil world. At night, when the glare of the sun passes out of our
+ atmosphere, the stars and planets seem to move across the heavens with a
+ stately and solemn slowness. It was one of the first discoveries of
+ modern astronomy that this movement is only apparent. The apparent
+ creeping of the stars across the heavens at night is accounted for by
+ the fact that the earth turns upon its axis once in every twenty-four
+ hours. When we remember the size of the earth we see that this implies a
+ prodigious speed.
+ In addition to this the earth revolves round the sun at a speed of more
+ than a thousand miles a minute. Its path round the sun, year in year
+ out, measures about 580,000,000 miles. The earth is held closely to this
+ path by the gravitational pull of the sun, which has a mass 333,432
+ times that of the earth. If at any moment the sun ceased to exert this
+ pull the earth would instantly fly off into space straight in the
+ direction in which it was moving at the time, that is to say, at a
+ tangent. This tendency to fly off at a tangent is continuous. It is the
+ balance between it and the sun's pull which keeps the earth to her
+ almost circular orbit. In the same way the seven other planets are held
+ to their orbits.
+ Circling round the earth, in the same way as the earth circles round the
+ sun, is our moon. Sometimes the moon passes directly between us and the
+ sun, and cuts off the light from us. We then have a total or partial
+ eclipse of the sun. At other times the earth passes directly between the
+ sun and the moon, and causes an eclipse of the moon. The great ball of
+ the earth naturally trails a mighty shadow across space, and the moon is
+ "eclipsed" when it passes into this.
+ The other seven planets, five of which have moons of their own, circle
+ round the sun as the earth does. The sun's mass is immensely larger than
+ that of all the planets put together, and all of them would be drawn
+ into it and perish if they did not travel rapidly round it in gigantic
+ orbits. So the eight planets, spinning round on their axes, follow their
+ fixed paths round the sun. The planets are secondary bodies, but they
+ are most important, because they are the only globes in which there can
+ be life, as we know life.
+ If we could be transported in some magical way to an immense distance in
+ space above the sun, we should see our Solar System as it is drawn in
+ the accompanying diagram (Fig. 1), except that the planets would be mere
+ specks, faintly visible in the light which they receive from the sun.
+ (This diagram is drawn approximately to scale.) If we moved still
+ farther away, trillions of miles away, the planets would fade entirely
+ out of view, and the sun would shrink into a point of fire, a star. And
+ here you begin to realize the nature of the universe. _The sun is a
+ star. The stars are suns._ Our sun looks big simply because of its
+ comparative nearness to us. The universe is a stupendous collection of
+ millions of stars or suns, many of which may have planetary families
+ like ours.
+ The Scale of the Universe
+ How many stars are there? A glance at a photograph of star-clouds will
+ tell at once that it is quite impossible to count them. The fine
+ photograph reproduced in Figure 2 represents a very small patch of that
+ pale-white belt, the Milky Way, which spans the sky at night. It is true
+ that this is a particularly rich area of the Milky Way, but the entire
+ belt of light has been resolved in this way into masses or clouds of
+ stars. Astronomers have counted the stars in typical districts here and
+ there, and from these partial counts we get some idea of the total
+ number of stars. There are estimated to be between two and three
+ thousand million stars.
+ Yet these stars are separated by inconceivable distances from each
+ other, and it is one of the greatest triumphs of modern astronomy to
+ have mastered, so far, the scale of the universe. For several centuries
+ astronomers have known the relative distances from each other of the sun
+ and the planets. If they could discover the actual distance of any one
+ planet from any other, they could at once tell all the distances within
+ the Solar System.
+ The sun is, on the latest measurements, at an average distance of
+ 92,830,000 miles from the earth, for as the orbit of the earth is not a
+ true circle, this distance varies. This means that in six months from
+ now the earth will be right at the opposite side of its path round the
+ sun, or 185,000,000 miles away from where it is now. Viewed or
+ photographed from two positions so wide apart, the nearest stars show a
+ tiny "shift" against the background of the most distant stars, and that
+ is enough for the mathematician. He can calculate the distance of any
+ star near enough to show this "shift." We have found that the nearest
+ star to the earth, a recently discovered star, is twenty-five trillion
+ miles away. Only thirty stars are known to be within a hundred trillion
+ miles of us.
+ This way of measuring does not, however, take us very far away in the
+ heavens. There are only a few hundred stars within five hundred trillion
+ miles of the earth, and at that distance the "shift" of a star against
+ the background (parallax, the astronomer calls it) is so minute that
+ figures are very uncertain. At this point the astronomer takes up a new
+ method. He learns the different types of stars, and then he is able to
+ deduce more or less accurately the distance of a star of a known type
+ from its faintness. He, of course, has instruments for gauging their
+ light. As a result of twenty years work in this field, it is now known
+ that the more distant stars of the Milky Way are at least a hundred
+ thousand trillion (100,000,000,000,000,000) miles away from the sun.
+ Our sun is in a more or less central region of the universe, or a few
+ hundred trillion miles from the actual centre. The remainder of the
+ stars, which are all outside our Solar System, are spread out,
+ apparently, in an enormous disc-like collection, so vast that even a ray
+ of light, which travels at the rate of 186,000 miles a second, would
+ take 50,000 years to travel from one end of it to the other. This, then
+ is what we call our universe.
+ Are there other Universes?
+ Why do we say "our universe"? Why not _the_ universe? It is now believed
+ by many of our most distinguished astronomers that our colossal family
+ of stars is only one of many universes. By a universe an astronomer
+ means any collection of stars which are close enough to control each
+ other's movements by gravitation; and it is clear that there might be
+ many universes, in this sense, separated from each other by profound
+ abysses of space. Probably there are.
+ For a long time we have been familiar with certain strange objects in
+ the heavens which are called "spiral nebulæ" (Fig 4). We shall see at a
+ later stage what a nebula is, and we shall see that some astronomers
+ regard these spiral nebulæ as worlds "in the making." But some of the
+ most eminent astronomers believe that they are separate
+ universes--"island-universes" they call them--or great collections of
+ millions of stars like our universe. There are certain peculiarities in
+ the structure of the Milky Way which lead these astronomers to think
+ that our universe may be a spiral nebula, and that the other spiral
+ nebulæ are "other universes."
+ [Illustration: _Photo: Harvard College Observatory._
+ FIG. 2.--THE MILKY WAY
+ Note the cloud-like effect.]
+ [Illustration: FIG. 3--THE MOON ENTERING THE SHADOW CAST BY THE EARTH
+ The diagram shows the Moon partially eclipsed.]
+ [Illustration: _From a photograph taken at the Yerkes Observatory_
+ FIG. 4.--THE GREAT NEBULA IN ANDROMEDA, MESSIER 31]
+ Vast as is the Solar System, then, it is excessively minute in
+ comparison with the Stellar System, the universe of the Stars, which is
+ on a scale far transcending anything the human mind can apprehend.
+ THE SOLAR SYSTEM
+ THE SUN
+ But now let us turn to the Solar System, and consider the members of our
+ own little colony.
+ Within the Solar System there are a large number of problems that
+ interest us. What is the size, mass, and distance of each of the
+ planets? What satellites, like our Moon, do they possess? What are their
+ temperatures? And those other, sporadic members of our system, comets
+ and meteors, what are they? What are their movements? How do they
+ originate? And the Sun itself, what is its composition, what is the
+ source of its heat, how did it originate? Is it running down?
+ These last questions introduce us to a branch of astronomy which is
+ concerned with the physical constitution of the stars, a study which,
+ not so very many years ago, may well have appeared inconceivable. But
+ the spectroscope enables us to answer even these questions, and the
+ answer opens up questions of yet greater interest. We find that the
+ stars can be arranged in an order of development--that there are stars
+ at all stages of their life-history. The main lines of the evolution of
+ the stellar universe can be worked out. In the sun and stars we have
+ furnaces with temperatures enormously high; it is in such conditions
+ that substances are resolved into their simplest forms, and it is thus
+ we are enabled to obtain a knowledge of the most primitive forms of
+ matter. It is in this direction that the spectroscope (which we shall
+ refer to immediately) has helped us so much. It is to this wonderful
+ instrument that we owe our knowledge of the composition of the sun and
+ stars, as we shall see.
+ "That the spectroscope will detect the millionth of a milligram of
+ matter, and on that account has discovered new elements, commands
+ our admiration; but when we find in addition that it will detect the
+ nature of forms of matter trillions of miles away, and moreover,
+ that it will measure the velocities with which these forms of matter
+ are moving with an absurdly small per cent. of possible error, we
+ can easily acquiesce in the statement that it is the greatest
+ instrument ever devised by the brain and hand of man."
+ Such are some of the questions with which modern astronomy deals. To
+ answer them requires the employment of instruments of almost incredible
+ refinement and exactitude and also the full resources of mathematical
+ genius. Whether astronomy be judged from the point of view of the
+ phenomena studied, the vast masses, the immense distances, the æons of
+ time, or whether it be judged as a monument of human ingenuity,
+ patience, and the rarest type of genius, it is certainly one of the
+ grandest, as it is also one of the oldest, of the sciences.
+ The Solar System
+ In the Solar System we include all those bodies dependent on the sun
+ which circulate round it at various distances, deriving their light and
+ heat from the sun--the planets and their moons, certain comets and a
+ multitude of meteors: in other words, all bodies whose movements in
+ space are determined by the gravitational pull of the sun.
+ The Sun
+ Thanks to our wonderful modern instruments and the ingenious methods
+ used by astronomers, we have to-day a remarkable knowledge of the sun.
+ Look at the figure of the sun in the frontispiece. The picture
+ represents an eclipse of the sun; the dark body of the moon has screened
+ the sun's shining disc and taken the glare out of our eyes; we see a
+ silvery halo surrounding the great orb on every side. It is the sun's
+ atmosphere, or "crown" (corona), stretching for millions of miles into
+ space in the form of a soft silvery-looking light; probably much of its
+ light is sunlight reflected from particles of dust, although the
+ spectroscope shows an element in the corona that has not so far been
+ detected anywhere else in the universe and which in consequence has been
+ named Coronium.
+ We next notice in the illustration that at the base of the halo there
+ are red flames peeping out from the edges of the hidden disc. When one
+ remembers that the sun is 866,000 miles in diameter, one hardly needs to
+ be told that these flames are really gigantic. We shall see what they
+ are presently.
+ Regions of the Sun
+ The astronomer has divided the sun into definite concentric regions or
+ layers. These layers envelop the nucleus or central body of the sun
+ somewhat as the atmosphere envelops our earth. It is through these
+ vapour layers that the bright white body of the sun is seen. Of the
+ innermost region, the heart or nucleus of the sun, we know almost
+ nothing. The central body or nucleus is surrounded by a brilliantly
+ luminous envelope or layer of vaporous matter which is what we see when
+ we look at the sun and which the astronomer calls the photosphere.
+ Above--that is, overlying--the photosphere there is a second layer of
+ glowing gases, which is known as the reversing layer. This layer is
+ cooler than the underlying photosphere; it forms a veil of smoke-like
+ haze and is of from 500 to 1,000 miles in thickness.
+ A third layer or envelope immediately lying over the last one is the
+ region known as the chromosphere. The chromosphere extends from 5,000
+ to 10,000 miles in thickness--a "sea" of red tumultuous surging fire.
+ Chief among the glowing gases is the vapour of hydrogen. The intense
+ white heat of the photosphere beneath shines through this layer,
+ overpowering its brilliant redness. From the uppermost portion of the
+ chromosphere great fiery tongues of glowing hydrogen and calcium vapour
+ shoot out for many thousands of miles, driven outward by some prodigious
+ expulsive force. It is these red "prominences" which are such a notable
+ feature in the picture of the eclipse of the sun already referred to.
+ During the solar eclipse of 1919 one of these red flames rose in less
+ than seven hours from a height of 130,000 miles to more than 500,000
+ miles above the sun's surface. This immense column of red-hot gas, four
+ or five times the thickness of the earth, was soaring upward at the rate
+ of 60,000 miles an hour.
+ These flaming jets or prominences shooting out from the chromosphere are
+ not to be seen every day by the naked eye; the dazzling light of the sun
+ obscures them, gigantic as they are. They can be observed, however, by
+ the spectroscope any day, and they are visible to us for a very short
+ time during an eclipse of the sun. Some extraordinary outbursts have
+ been witnessed. Thus the late Professor Young described one on September
+ 7, 1871, when he had been examining a prominence by the spectroscope:
+ It had remained unchanged since noon of the previous day--a long,
+ low, quiet-looking cloud, not very dense, or brilliant, or in any
+ way remarkable except for its size. At 12:30 p.m. the Professor left
+ the spectroscope for a short time, and on returning half an hour
+ later to his observations, he was astonished to find the gigantic
+ Sun flame shattered to pieces. The solar atmosphere was filled with
+ flying debris, and some of these portions reached a height of
+ 100,000 miles above the solar surface. Moving with a velocity which,
+ even at the distance of 93,000,000 miles, was almost perceptible to
+ the eye, these fragments doubled their height in ten minutes. On
+ January 30, 1885, another distinguished solar observer, the late
+ Professor Tacchini of Rome, observed one of the greatest prominences
+ ever seen by man. Its height was no less than 142,000
+ miles--eighteen times the diameter of the earth. Another mighty
+ flame was so vast that supposing the eight large planets of the
+ solar system ranged one on top of the other, the prominence would
+ still tower above them.[1]
+ [1] _The Romance of Astronomy_, by H. Macpherson.
+ [Illustration: FIG. 5.--DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN
+ Compare with frontispiece.]
+ [Illustration: _Photo: Royal Observatory, Greenwich._
+ FIG. 6.--SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, May 29, 1919.
+ TAKEN AT SOBRAL, BRAZIL.
+ The small Corona is also visible.]
+ [Illustration: FIG. 7.--THE VISIBLE SURFACE OF THE SUN
+ A photograph taken at the Mount Wilson Observatory of the Carnegie
+ Institution at Washington.]
+ [Illustration: FIG. 8.--THE SUN
+ Photographed in the light of glowing hydrogen, at the Mount Wilson
+ Observatory of the Carnegie Institution of Washington: vortex phenomena
+ near the spots are especially prominent.]
+ The fourth and uppermost layer or region is that of the corona, of
+ immense extent and fading away into the surrounding sky--this we have
+ already referred to. The diagram (Fig. 5) shows the dispositions of
+ these various layers of the sun. It is through these several transparent
+ layers that we see the white light body of the sun.
+ The Surface of the Sun
+ Here let us return to and see what more we know about the
+ photosphere--the sun's surface. It is from the photosphere that we have
+ gained most of our knowledge of the composition of the sun, which is
+ believed not to be a solid body. Examination of the photosphere shows
+ that the outer surface is never at rest. Small bright cloudlets come and
+ go in rapid succession, giving the surface, through contrasts in
+ luminosity, a granular appearance. Of course, to be visible at all at
+ 92,830,000 miles the cloudlets cannot be small. They imply enormous
+ activity in the photosphere. If we might speak picturesquely the sun's
+ surface resembles a boiling ocean of white-hot metal vapours. We have
+ to-day a wonderful instrument, which will be described later, which
+ dilutes, as it were, the general glare of the sun, and enables us to
+ observe these fiery eruptions at any hour. The "oceans" of red-hot gas
+ and white-hot metal vapour at the sun's surface are constantly driven by
+ great storms. Some unimaginable energy streams out from the body or
+ muscles of the sun and blows its outer layers into gigantic shreds, as
+ it were.
+ The actual temperature at the sun's surface, or what appears to us to be
+ the surface--the photosphere--is, of course, unknown, but careful
+ calculation suggests that it is from 5,000° C. to 7,000° C. The interior
+ is vastly hotter. We can form no conception of such temperatures as must
+ exist there. Not even the most obdurate solid could resist such
+ temperatures, but would be converted almost instantaneously into gas.
+ But it would not be gas as we know gases on the earth. The enormous
+ pressures that exist on the sun must convert even gases into thick
+ treacly fluids. We can only infer this state of matter. It is beyond our
+ power to reproduce it.
+ Sun-spots
+ It is in the brilliant photosphere that the dark areas known as
+ sun-spots appear. Some of these dark spots--they are dark only by
+ contrast with the photosphere surrounding them--are of enormous size,
+ covering many thousands of square miles of surface. What they are we
+ cannot positively say. They look like great cavities in the sun's
+ surface. Some think they are giant whirlpools. Certainly they seem to be
+ great whirling streams of glowing gases with vapours above them and
+ immense upward and downward currents within them. Round the edges of the
+ sun-spots rise great tongues of flame.
+ Perhaps the most popularly known fact about sun-spots is that they are
+ somehow connected with what we call magnetic storms on earth. These
+ magnetic storms manifest themselves in interruptions of our telegraphic
+ and telephonic communications, in violent disturbances of the mariner's
+ compass, and in exceptional auroral displays. The connection between the
+ two sets of phenomena cannot be doubted, even although at times there
+ may be a great spot on the sun without any corresponding "magnetic
+ storm" effects on the earth.
+ A surprising fact about sun-spots is that they show definite periodic
+ variations in number. The best-defined period is one of about eleven
+ years. During this period the spots increase to a maximum in number and
+ then diminish to a minimum, the variation being more or less regular.
+ Now this can only mean one thing. To be periodic the spots must have
+ some deep-seated connection with the fundamental facts of the sun's
+ structure and activities. Looked at from this point of view their
+ importance becomes great.
+ [Illustration: _Reproduction from "The Forces of Nature"_ (_Messrs.
+ Macmillan_)
+ THE AURORA BOREALIS
+ The aurora borealis is one of the most beautiful spectacles in the sky.
+ The colours and shape change every instant; sometimes a fan-like cluster
+ of rays, at other times long golden draperies gliding one over the
+ other. Blue, green, yellow, red, and white combine to give a glorious
+ display of colour. The theory of its origin is still, in part, obscure,
+ but there can be no doubt that the aurora is related to the magnetic
+ phenomena of the earth and therefore is connected with the electrical
+ influence of the sun.]
+ It is from the study of sun-spots that we have learned that the sun's
+ surface does not appear to rotate all at the same speed. The
+ "equatorial" regions are rotating quicker than regions farther north or
+ south. A point forty-five degrees from the equator seems to take about
+ two and a half days longer to complete one rotation than a point on the
+ equator. This, of course, confirms our belief that the sun cannot be a
+ solid body.
+ What is its composition? We know that there are present, in a gaseous
+ state, such well-known elements as sodium, iron, copper, zinc, and
+ magnesium; indeed, we know that there is practically every element in
+ the sun that we know to be in the earth. How do we know?
+ It is from the photosphere, as has been said, that we have won most of
+ our knowledge of the sun. The instrument used for this purpose is the
+ spectroscope; and before proceeding to deal further with the sun and the
+ source of its energy it will be better to describe this instrument.
+ A WONDERFUL INSTRUMENT AND WHAT IT REVEALS
+ The spectroscope is an instrument for analysing light. So important is
+ it in the revelations it has given us that it will be best to describe
+ it fully. Every substance to be examined must first be made to glow,
+ made luminous; and as nearly everything in the heavens _is_ luminous the
+ instrument has a great range in Astronomy. And when we speak of
+ analysing light, we mean that the light may be broken up into waves of
+ different lengths. What we call light is a series of minute waves in
+ ether, and these waves are--measuring them from crest to crest, so to
+ say--of various lengths. Each wave-length corresponds to a colour of the
+ rainbow. The shortest waves give us a sensation of violet colour, and
+ the largest waves cause a sensation of red. The rainbow, in fact, is a
+ sort of natural spectrum. (The meaning of the rainbow is that the
+ moisture-laden air has sorted out these waves, in the sun's light,
+ according to their length.) Now the simplest form of spectroscope is a
+ glass prism--a triangular-shaped piece of glass. If white light
+ (sunlight, for example) passes through a glass prism, we see a series of
+ rainbow-tinted colours. Anyone can notice this effect when sunlight is
+ shining through any kind of cut glass--the stopper of a wine decanter,
+ for instance. If, instead of catching with the eye the coloured lights
+ as they emerge from the glass prism, we allow them to fall on a screen,
+ we shall find that they pass, by continuous gradations, from red at the
+ one end of the screen, through orange, yellow, green, blue, and indigo,
+ to violet at the other end. _In other words, what we call white light is
+ composed of rays of these several colours. They go to make up the effect
+ which we call white._ And now just as water can be split up into its two
+ elements, oxygen and hydrogen, so sunlight can be broken up into its
+ primary colours, which are those we have just mentioned.
+ This range of colours, produced by the spectroscope, we call the solar
+ spectrum, and these are, from the spectroscopic point of view, primary
+ colours. Each shade of colour has its definite position in the spectrum.
+ That is to say, the light of each shade of colour (corresponding to its
+ wave-length) is reflected through a certain fixed angle on passing
+ through the glass prism. Every possible kind of light has its definite
+ position, and is denoted by a number which gives the wave-length of the
+ vibrations constituting that particular kind of light.
+ Now, other kinds of light besides sunlight can be analysed. Light
+ from any substance which has been made incandescent may be observed with
+ the spectroscope in the same way, and each element can be thus
+ separated. It is found that each substance (in the same conditions of
+ pressure, etc.) gives a constant spectrum of its own. _Each metal
+ displays its own distinctive colour. It is obvious, therefore, that the
+ spectrum provides the means for identifying a particular substance._ It
+ was by this method that we discovered in the sun the presence of such
+ well-known elements as sodium, iron, copper, zinc, and magnesium.
+ [Illustration: _Yerkes Observatory._
+ FIG. 9.--THE GREAT SUN-SPOT OF JULY 17, 1905]
+ [Illustration: _From photographs taken at the Yerkes Observatory._
+ FIG. 10.--SOLAR PROMINENCES
+ These are about 60,000 miles in height. The two photographs show the
+ vast changes occurring in ten minutes. October 10, 1910.]
+ [Illustration: _Photo: Mount Wilson Observatory._
+ FIG. 11.--MARS, October 5, 1909
+ Showing the dark markings and the Polar Cap.]
+ [Illustration: FIG. 12.--JUPITER
+ Showing the belts which are probably cloud formations.]
+ [Illustration: _Photo: Professor E. E. Barnard, Yerkes Observatory._
+ FIG. 13.--SATURN, November 19, 1911
+ Showing the rings, mighty swarms of meteorites.]
+ Every chemical element known, then, has a distinctive spectrum of its
+ own when it is raised to incandescence, and this distinctive spectrum is
+ as reliable a means of identification for the element as a human face is
+ for its owner. Whether it is a substance glowing in the laboratory or in
+ a remote star makes no difference to the spectroscope; if the light of
+ any substance reaches it, that substance will be recognised and
+ identified by the characteristic set of waves.
+ The spectrum of a glowing mass of gas will consist in a number of bright
+ lines of various colours, and at various intervals; corresponding to
+ each kind of gas, there will be a peculiar and distinctive arrangement
+ of bright lines. But if the light from such a mass of glowing gas be
+ made to pass through a cool mass of the _same_ gas it will be found that
+ dark lines replace the bright lines in the spectrum, the reason for this
+ being that the cool gas absorbs the rays of light emitted by the hot
+ gas. Experiments of this kind enable us to reach the important general
+ statement that every gas, when cold, absorbs the same rays of light
+ which it emits when hot.
+ Crossing the solar spectrum are hundreds and hundreds of dark lines.
+ These could not at first be explained, because this fact of
+ discriminative absorption was not known. We understand now. The sun's
+ white light comes from the photosphere, but between us and the
+ photosphere there is, as we have seen, another solar envelope of
+ relatively cooler vapours--the reversing layer. Each constituent
+ element in this outer envelope stops its own kind of light, that is, the
+ kind of light made by incandescent atoms of the same element in the
+ photosphere. The "stoppages" register themselves in the solar spectrum
+ as dark lines placed exactly where the corresponding bright lines would
+ have been. The explanation once attained, dark lines became as
+ significant as bright lines. The secret of the sun's composition was
+ out. We have found practically every element in the sun that we know to
+ be in the earth. We have identified an element in the sun before we were
+ able to isolate it on the earth. We have been able even to point to the
+ coolest places on the sun, the centres of sun-spots, where alone the
+ temperature seems to have fallen sufficiently low to allow chemical
+ compounds to form.
+ It is thus we have been able to determine what the stars, comets, or
+ nebulæ are made of.
+ A Unique Discovery
+ In 1868 Sir Norman Lockyer detected a light coming from the prominences
+ of the sun which was not given by any substance known on earth, and
+ attributed this to an unknown gas which he called helium, from the Greek
+ _helios_, the sun. _In 1895 Sir William Ramsay discovered in certain
+ minerals the same gas identified by the spectroscope._ We can say,
+ therefore, that this gas was discovered in the sun nearly thirty years
+ before it was found on earth; this discovery of the long-lost heir is as
+ thrilling a chapter in the detective story of science as any in the
+ sensational stories of the day, and makes us feel quite certain that our
+ methods really tell us of what elements sun and stars are built up. The
+ light from the corona of the sun, as we have mentioned indicates a gas
+ still unknown on earth, which has been christened Coronium.
+ Measuring the Speed of Light
+ But this is not all; soon a new use was found for the spectroscope. We
+ found that we could measure with it the most difficult of all speeds
+ to measure, speed in the line of sight. Movement at right angles to the
+ direction in which one is looking is, if there is sufficient of it, easy
+ to detect, and, if the distance of the moving body is known, easy to
+ measure. But movement in the line of vision is both difficult to detect
+ and difficult to measure. Yet, even at the enormous distances with which
+ astronomers have to deal, the spectroscope can detect such movement and
+ furnish data for its measurement. If a luminous body containing, say,
+ sodium is moving rapidly towards the spectroscope, it will be found that
+ the sodium lines in the spectrum have moved slightly from their usual
+ definite positions towards the violet end of the spectrum, the amount of
+ the change of position increasing with the speed of the luminous body.
+ If the body is moving away from the spectroscope the shifting of the
+ spectral lines will be in the opposite direction, towards the red end of
+ the spectrum. In this way we have discovered and measured movements that
+ otherwise would probably not have revealed themselves unmistakably to us
+ for thousands of years. In the same way we have watched, and measured
+ the speed of, tremendous movements on the sun, and so gained proof that
+ the vast disturbances we should expect there actually do occur.
+ [Illustration: THE SPECTROSCOPE IS AN INSTRUMENT FOR ANALYSING LIGHT; IT
+ PROVIDES THE MEANS FOR IDENTIFYING DIFFERENT SUBSTANCES
+ This pictorial diagram illustrates the principal of Spectrum Analysis,
+ showing how sunlight is decomposed into its primary colours. What we
+ call white light is composed of seven different colours. The diagram is
+ relieved of all detail which would unduly obscure the simple process by
+ which a ray of light is broken up by a prism into different
+ wave-lengths. The spectrum rays have been greatly magnified.]
+ IS THE SUN DYING?
+ Now let us return to our consideration of the sun.
+ To us on the earth the most patent and most astonishing fact about the
+ sun is its tremendous energy. Heat and light in amazing quantities pour
+ from it without ceasing.
+ Where does this energy come from? Enormous jets of red glowing gases can
+ be seen shooting outwards from the sun, like flames from a fire, for
+ thousands of miles. Does this argue fire, as we know fire on the earth?
+ On this point the scientist is sure. The sun is not burning, and
+ combustion is not the source of its heat. Combustion is a chemical
+ reaction between atoms. The conditions that make it possible are known
+ and the results are predictable and measurable. But no chemical reaction
+ of the nature of combustion as we know it will explain the sun's energy,
+ nor indeed will any ordinary chemical reaction of any kind. If the sun
+ were composed of combustible material throughout and the conditions of
+ combustion as we understand them were always present, the sun would burn
+ itself out in some thousands of years, with marked changes in its heat
+ and light production as the process advanced. There is no evidence of
+ such changes. There is, instead, strong evidence that the sun has been
+ emitting light and heat in prodigious quantities, not for thousands, but
+ for millions of years. Every addition to our knowledge that throws light
+ on the sun's age seems to make for increase rather than decrease of its
+ years. This makes the wonder of its energy greater.
+ And we cannot avoid the issue of the source of the energy by saying
+ merely that the sun is gradually radiating away an energy that
+ originated in some unknown manner, away back at the beginning of things.
+ Reliable calculations show that the years required for the mere cooling
+ of a globe like the sun could not possibly run to millions. In other
+ words, the sun's energy must be subject to continuous and more or less
+ steady renewal. However it may have acquired its enormous energy in the
+ past, it must have some source of energy in the present.
+ The best explanation that we have to-day of this continuous accretion of
+ energy is that it is due to shrinkage of the sun's bulk under the force
+ of gravity. Gravity is one of the most mysterious forces of nature, but
+ it is an obvious fact that bodies behave as if they attracted one
+ another, and Newton worked out the law of this attraction. We may say,
+ without trying to go too deeply into things, that every particle of
+ matter attracts every other throughout the universe. If the diameter of
+ the sun were to shrink by one mile all round, this would mean that all
+ the millions of tons in the outer one-mile thickness would have a
+ straight drop of one mile towards the centre. And that is not all,
+ because obviously the layers below this outer mile would also drop
+ inwards, each to a less degree than the one above it. What a tremendous
+ movement of matter, however slowly it might take place! And what a
+ tremendous energy would be involved! Astronomers calculate that the
+ above shrinkage of one mile all round would require fifty years for its
+ completion, assuming, reasonably, that there is close and continuous
+ relationship between loss of heat by radiation and shrinkage. Even if
+ this were true we need not feel over-anxious on this theory; before the
+ sun became too cold to support life many millions of years would be
+ required.
+ It was suggested at one time that falls of meteoric matter into the sun
+ would account for the sun's heat. This position is hardly tenable now.
+ The mere bulk of the meteoric matter required by the hypothesis, apart
+ from other reasons, is against it. There is undoubtedly an enormous
+ amount of meteoric matter moving about within the bounds of the solar
+ system, but most of it seems to be following definite routes round the
+ sun like the planets. The stray erratic quantities destined to meet
+ their doom by collision with the sun can hardly be sufficient to account
+ for the sun's heat.
+ Recent study of radio-active bodies has suggested another factor that
+ may be working powerfully along with the force of gravitation to
+ maintain the sun's store of heat. In radio-active bodies certain atoms
+ seem to be undergoing disintegration. These atoms appear to be splitting
+ up into very minute and primitive constituents. But since matter may be
+ split up into such constituents, may it not be built up from them?
+ The question is whether these "radio-active" elements are undergoing
+ disintegration, or formation, in the sun. If they are undergoing
+ disintegration--and the sun itself is undoubtedly radio-active--then we
+ have another source of heat for the sun that will last indefinitely.
+ THE PLANETS
+ LIFE IN OTHER WORLDS?
+ It is quite clear that there cannot be life on the stars. Nothing solid
+ or even liquid can exist in such furnaces as they are. Life exists only
+ on planets, and even on these its possibilities are limited. Whether all
+ the stars, or how many of them, have planetary families like our sun, we
+ cannot positively say. If they have, such planets would be too faint and
+ small to be visible tens of trillions of miles away. Some astronomers
+ think that our sun may be exceptional in having planets, but their
+ reasons are speculative and unconvincing. Probably a large proportion at
+ least of the stars have planets, and we may therefore survey the globes
+ of our own solar system and in a general way extend the results to the
+ rest of the universe.
+ In considering the possibility of life as we know it we may at once rule
+ out the most distant planets from the sun, Uranus and Neptune. They are
+ probably intrinsically too hot. We may also pass over the nearest planet
+ to the sun, Mercury. We have reason to believe that it turns on its axis
+ in the same period as it revolves round the sun, and it must therefore
+ always present the same side to the sun. This means that the heat on the
+ sunlit side of Mercury is above boiling-point, while the cold on the
+ other side must be between two and three hundred degrees below
+ freezing-point.
+ The Planet Venus
+ The planet Venus, the bright globe which is known to all as the morning
+ and evening "star," seems at first sight more promising as regards the
+ possibility of life. It is of nearly the same size as the earth, and it
+ has a good atmosphere, but there are many astronomers who believe that,
+ like Mercury, it always presents the same face to the sun, and it would
+ therefore have the same disadvantage--a broiling heat on the sunny side
+ and the cold of space on the opposite side. We are not sure. The
+ surface of Venus is so bright--the light of the sun is reflected to us
+ by such dense masses of cloud and dust--that it is difficult to trace
+ any permanent markings on it, and thus ascertain how long it takes to
+ rotate on its axis. Many astronomers believe that they have succeeded,
+ and that the planet always turns the same face to the sun. If it does,
+ we can hardly conceive of life on its surface, in spite of the
+ cloud-screen.
+ [Illustration: FIG. 14.--THE MOON
+ Showing a great plain and some typical craters. There are thousands of
+ these craters, and some theories of their origin are explained on page
+ 34.]
+ [Illustration: FIG. 15.--MARS
+ 1} Drawings by Prof. Lowell to accompany actual photographs of Mars
+ showing many of the
+ 2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory.
+ 3 Drawing by Prof. Lowell made January 6, 1914.
+ 4 Drawing by Prof. Lowell made January 21, 1914.
+ Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4
+ depict quite different sections. Note the change in the polar snow-caps
+ in the last two.]
+ [Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYS
+ Note the mysterious "rays" diverging from the almost perfectly circular
+ craters indicated by the arrows (Tycho, upper; Copernicus, lower), and
+ also the mountains to the right with the lunar dawn breaking on them.]
+ We turn to Mars; and we must first make it clear why there is so much
+ speculation about life on Mars, and why it is supposed that, if there
+ _is_ life on Mars, it must be more advanced than life on the earth.
+ Is there Life on Mars?
+ The basis of this belief is that if, as we saw, all the globes in our
+ solar system are masses of metal that are cooling down, the smaller will
+ have cooled down before the larger, and will be further ahead in their
+ development. Now Mars is very much smaller than the earth, and must have
+ cooled at its surface millions of years before the earth did. Hence, if
+ a story of life began on Mars at all, it began long before the story of
+ life on the earth. We cannot guess what sort of life-forms would be
+ evolved in a different world, but we can confidently say that they would
+ tend toward increasing intelligence; and thus we are disposed to look
+ for highly intelligent beings on Mars.
+ But this argument supposes that the conditions of life, namely air and
+ water, are found on Mars, and it is disputed whether they are found
+ there in sufficient quantity. The late Professor Percival Lowell, who
+ made a lifelong study of Mars, maintained that there are hundreds of
+ straight lines drawn across the surface of the planet, and he claimed
+ that they are beds of vegetation marking the sites of great channels or
+ pipes by means of which the "Martians" draw water from their polar
+ ocean. Professor W. H. Pickering, another high authority, thinks that
+ the lines are long, narrow marshes fed by moist winds from the poles.
+ There are certainly white polar caps on Mars. They seem to melt in the
+ spring, and the dark fringe round them grows broader.
+ Other astronomers, however, say that they find no trace of water-vapour
+ in the atmosphere of Mars, and they think that the polar caps may be
+ simply thin sheets of hoar-frost or frozen gas. They point out that, as
+ the atmosphere of Mars is certainly scanty, and the distance from the
+ sun is so great, it may be too cold for the fluid water to exist on the
+ planet.
+ If one asks why our wonderful instruments cannot settle these points,
+ one must be reminded that Mars is never nearer than 34,000,000 miles
+ from the earth, and only approaches to this distance once in fifteen or
+ seventeen years. The image of Mars on the photographic negative taken in
+ a big telescope is very small. Astronomers rely to a great extent on the
+ eye, which is more sensitive than the photographic plate. But it is easy
+ to have differences of opinion as to what the eye sees, and so there is
+ a good deal of controversy.
+ In August, 1924, the planet will again be well placed for observation,
+ and we may learn more about it. Already a few of the much-disputed
+ lines, which people wrongly call "canals," have been traced on
+ photographs. Astronomers who are sceptical about life on Mars are often
+ not fully aware of the extraordinary adaptability of life. There was a
+ time when the climate of the whole earth, from pole to pole, was
+ semi-tropical for millions of years. No animal could then endure the
+ least cold, yet now we have plenty of Arctic plants and animals. If the
+ cold came slowly on Mars, as we have reason to suppose, the population
+ could be gradually adapted to it. On the whole, it is possible that
+ there is advanced life on Mars, and it is not impossible, in spite of
+ the very great difficulties of a code of communication, that our "elder
+ brothers" may yet flash across space the solution of many of our
+ problems.
+ Jupiter and Saturn
+ Next to Mars, going outward from the sun, is Jupiter. Between Mars and
+ Jupiter, however, there are more than three hundred million miles of
+ space, and the older astronomers wondered why this was not occupied by a
+ planet. We now know that it contains about nine hundred "planetoids," or
+ small globes of from five to five hundred miles in diameter. It was at
+ one time thought that a planet might have burst into these fragments (a
+ theory which is not mathematically satisfactory), or it may be that the
+ material which is scattered in them was prevented by the nearness of the
+ great bulk of Jupiter from uniting into one globe.
+ For Jupiter is a giant planet, and its gravitational influence must
+ extend far over space. It is 1,300 times as large as the earth, and has
+ nine moons, four of which are large, in attendance on it. It is
+ interesting to note that the outermost moons of Jupiter and Saturn
+ revolve round these planets in a direction contrary to the usual
+ direction taken by moons round planets, and by planets round the sun.
+ But there is no life on Jupiter.
+ The surface which we see in photographs (Fig. 12) is a mass of cloud or
+ steam which always envelops the body of the planet. It is apparently
+ red-hot. A red tinge is seen sometimes at the edges of its cloud-belts,
+ and a large red region (the "red spot"), 23,000 miles in length, has
+ been visible on it for half a century. There may be a liquid or solid
+ core to the planet, but as a whole it is a mass of seething vapours
+ whirling round on its axis once in every ten hours. As in the case of
+ the sun, however, different latitudes appear to rotate at different
+ rates. The interior of Jupiter is very hot, but the planet is not
+ self-luminous. The planets Venus and Jupiter shine very brightly, but
+ they have no light of their own; they reflect the sunlight.
+ Saturn is in the same interesting condition. The surface in the
+ photograph (Fig. 13) is steam, and Saturn is so far away from the sun
+ that the vaporisation of its oceans must necessarily be due to its own
+ internal heat. It is too hot for water to settle on its surface. Like
+ Jupiter, the great globe turns on its axis once in ten hours--a
+ prodigious speed--and must be a swirling, seething mass of metallic
+ vapours and gases. It is instructive to compare Jupiter and Saturn in
+ this respect with the sun. They are smaller globes and have cooled down
+ more than the central fire.
+ Saturn is a beautiful object in the telescope because it has ten moons
+ (to include one which is disputed) and a wonderful system of "rings"
+ round it. The so-called rings are a mighty swarm of meteorites--pieces
+ of iron and stone of all sorts and sizes, which reflect the light of the
+ sun to us. This ocean of matter is some miles deep, and stretches from a
+ few thousand miles from the surface of the planet to 172,000 miles out
+ in space. Some astronomers think that this is volcanic material which
+ has been shot out of the planet. Others regard it as stuff which would
+ have combined to form an eleventh moon but was prevented by the nearness
+ of Saturn itself. There is no evidence of life on Saturn.
+ THE MOON
+ Mars and Venus are therefore the only planets, besides the earth, on
+ which we may look for life; and in the case of Venus, the possibility is
+ very faint. But what about the moons which attend the planets? They
+ range in size from the little ten-miles-wide moons of Mars, to Titan, a
+ moon of Saturn, and Ganymede, a satellite of Jupiter, which are about
+ 3,000 miles in diameter. May there not be life on some of the larger of
+ these moons? We will take our own moon as a type of the class.
+ A Dead World
+ The moon is so very much nearer to us than any other heavenly body that
+ we have a remarkable knowledge of it. In Fig. 14 you have a photograph,
+ taken in one of our largest telescopes, of part of its surface. In a
+ sense such a telescope brings the moon to within about fifty miles of
+ us. We should see a city like London as a dark, sprawling blotch on the
+ globe. We could just detect a Zeppelin or a Diplodocus as a moving speck
+ against the surface. But we find none of these things. It is true that a
+ few astronomers believe that they see signs of some sort of feeble life
+ or movement on the moon. Professor Pickering thinks that he can trace
+ some volcanic activity. He believes that there are areas of vegetation,
+ probably of a low order, and that the soil of the moon may retain a
+ certain amount of water in it. He speaks of a very thin atmosphere, and
+ of occasional light falls of snow. He has succeeded in persuading some
+ careful observers that there probably are slight changes of some kind
+ taking place on the moon.
+ [Illustration: FIG. 17.--A MAP OF THE CHIEF PLAINS AND CRATERS OF THE
+ MOON
+ The plains were originally supposed to be seas: hence the name "Mare."]
+ [Illustration: FIG. 18.--A DIAGRAM OF A STREAM OF METEORS SHOWING THE
+ EARTH PASSING THROUGH THEM] [Illustration: _Photo: Royal Observatory,
+ Greenwich._
+ FIG. 19.--COMET, September 29, 1908
+ Notice the tendency to form a number of tails. (See photograph below.)]
+ [Illustration: _Photo: Royal Observatory, Greenwich._
+ FIG. 20.--COMET, October 3, 1908
+ The process has gone further and a number of distinct tails can now be
+ counted.]
+ But there are many things that point to absence of air on the moon. Even
+ the photographs we reproduce tell the same story. The edges of the
+ shadows are all hard and black. If there had been an appreciable
+ atmosphere it would have scattered the sun's light on to the edges and
+ produced a gradual shading off such as we see on the earth. This
+ relative absence of air must give rise to some surprising effects. There
+ will be no sounds on the moon, because sounds are merely air waves. Even
+ a meteor shattering itself to a violent end against the surface of the
+ moon would make no noise. Nor would it herald its coming by glowing into
+ a "shooting star," as it would on entering the earth's atmosphere. There
+ will be no floating dust, no scent, no twilight, no blue sky, no
+ twinkling of the stars. The sky will be always black and the stars will
+ be clearly visible by day as by night. The sun's wonderful corona, which
+ no man on earth, even by seizing every opportunity during eclipses, can
+ hope to see for more than two hours in all in a long lifetime, will be
+ visible all day. So will the great red flames of the sun. Of course,
+ there will be no life, and no landscape effects and scenery effects due
+ to vegetation.
+ The moon takes approximately twenty-seven of our days to turn once on
+ its axis. So for fourteen days there is continuous night, when the
+ temperature must sink away down towards the absolute cold of space. This
+ will be followed without an instant of twilight by full daylight. For
+ another fourteen days the sun's rays will bear straight down, with no
+ diffusion or absorption of their heat, or light, on the way. It does not
+ follow, however, that the temperature of the moon's surface must rise
+ enormously. It may not even rise to the temperature of melting ice.
+ Seeing there is no air there can be no check on radiation. The heat that
+ the moon gets will radiate away immediately. We know that amongst the
+ coldest places on the earth are the tops of very high mountains, the
+ points that have reared themselves nearest to the sun but farthest out
+ of the sheltering blanket of the earth's atmosphere. The actual
+ temperature of the moon's surface by day is a moot point. It may be
+ below the freezing-point or above the boiling-point of water.
+ The Mountains of the Moon
+ The lack of air is considered by many astronomers to furnish the
+ explanation of the enormous number of "craters" which pit the moon's
+ surface. There are about a hundred thousand of these strange rings, and
+ it is now believed by many that they are spots where very large
+ meteorites, or even planetoids, splashed into the moon when its surface
+ was still soft. Other astronomers think that they are the remains of
+ gigantic bubbles which were raised in the moon's "skin," when the globe
+ was still molten, by volcanic gases from below. A few astronomers think
+ that they are, as is popularly supposed, the craters of extinct
+ volcanoes. Our craters, on the earth, are generally deep cups, whereas
+ these ring-formations on the moon are more like very shallow and broad
+ saucers. Clavius, the largest of them, is 123 miles across the interior,
+ yet its encircling rampart is not a mile high.
+ The mountains on the moon (Fig. 16) rise to a great height, and are
+ extraordinarily gaunt and rugged. They are like fountains of lava,
+ rising in places to 26,000 and 27,000 feet. The lunar Apennines have
+ three thousand steep and weird peaks. Our terrestrial mountains are
+ continually worn down by frost acting on moisture and by ice and water,
+ but there are none of these agencies operating on the moon. Its
+ mountains are comparatively "everlasting hills."
+ The moon is interesting to us precisely because it is a dead world. It
+ seems to show how the earth, or any cooling metal globe, will evolve in
+ the remote future. We do not know if there was ever life on the moon,
+ but in any case it cannot have proceeded far in development. At the most
+ we can imagine some strange lowly forms of vegetation lingering here and
+ there in pools of heavy gas, expanding during the blaze of the sun's
+ long day, and frozen rigid during the long night.
+ METEORS AND COMETS
+ We may conclude our survey of the solar system with a word about
+ "shooting stars," or meteors, and comets. There are few now who do not
+ know that the streak of fire which suddenly lights the sky overhead at
+ night means that a piece of stone or iron has entered our atmosphere
+ from outer space, and has been burned up by friction. It was travelling
+ at, perhaps, twenty or thirty miles a second. At seventy or eighty miles
+ above our heads it began to glow, as at that height the air is thick
+ enough to offer serious friction and raise it to a white heat. By the
+ time the meteor reached about twenty miles or so from the earth's
+ surface it was entirely dissipated, as a rule in fiery vapour.
+ Millions of Meteorites
+ It is estimated that between ten and a hundred million meteorites enter
+ our atmosphere and are cremated, every day. Most of them weigh only an
+ ounce or two, and are invisible. Some of them weigh a ton or more, but
+ even against these large masses the air acts as a kind of "torpedo-net."
+ They generally burst into fragments and fall without doing damage.
+ It is clear that "empty space" is, at least within the limits of our
+ solar system, full of these things. They swarm like fishes in the seas.
+ Like the fishes, moreover, they may be either solitary or gregarious.
+ The solitary bit of cosmic rubbish is the meteorite, which we have just
+ examined. A "social" group of meteorites is the essential part of a
+ comet. The nucleus, or bright central part, of the head of a comet (Fig.
+ 19) consists of a swarm, sometimes thousands of miles wide, of these
+ pieces of iron or stone. This swarm has come under the sun's
+ gravitational influence, and is forced to travel round it. From some
+ dark region of space it has moved slowly into our system. It is not then
+ a comet, for it has no tail. But as the crowded meteors approach the
+ sun, the speed increases. They give off fine vapour-like matter and the
+ fierce flood of light from the sun sweeps this vapour out in an
+ ever-lengthening tail. Whatever way the comet is travelling, the tail
+ always points away from the sun.
+ A Great Comet
+ The vapoury tail often grows to an enormous length as the comet
+ approaches the sun. The great comet of 1843 had a tail two hundred
+ million miles long. It is, however, composed of the thinnest vapours
+ imaginable. Twice during the nineteenth century the earth passed through
+ the tail of a comet, and nothing was felt. The vapours of the tail are,
+ in fact, so attenuated that we can hardly imagine them to be white-hot.
+ They may be lit by some electrical force. However that may be, the comet
+ dashes round the sun, often at three or four hundred miles a second,
+ then may pass gradually out of our system once more. It may be a
+ thousand years, or it may be fifty years, before the monarch of the
+ system will summon it again to make its fiery journey round his throne.
+ [Illustration: _Photo: Harvard College Observatory._
+ FIG. 21.--TYPICAL SPECTRA
+ Six main types of stellar spectra. Notice the lines they have in common,
+ showing what elements are met with in different types of stars. Each of
+ these spectra corresponds to a different set of physical and chemical
+ conditions.] [Illustration: _Photo: Mount Wilson Observatory._
+ FIG. 22.--A NEBULAR REGION SOUTH OF ZETA ORIONIS
+ Showing a great projection of "dark matter" cutting off the light from
+ behind.]
+ [Illustration: _Photo: Astrophysical Observatory, Victoria, British
+ Columbia._
+ FIG. 23.--STAR CLUSTER IN HERCULES
+ A wonderful cluster of stars. It has been estimated that the distance of
+ this cluster is such that it would take light more than 100,000 years to
+ reach us.]
+ THE STELLAR UNIVERSE
+ The immensity of the Stellar Universe, as we have seen, is beyond our
+ apprehension. The sun is nothing more than a very ordinary star, perhaps
+ an insignificant one. There are stars enormously greater than the sun.
+ One such, Betelgeux, has recently been measured, and its diameter is
+ more than 300 times that of the sun.
+ The Evolution of Stars
+ The proof of the similarity between our sun and the stars has come to us
+ through the spectroscope. The elements that we find by its means in the
+ sun are also found in the same way in the stars. Matter, says the
+ spectroscope, is essentially the same everywhere, in the earth and the
+ sun, in the comet that visits us once in a thousand years, in the star
+ whose distance is incalculable, and in the great clouds of "fire-mist"
+ that we call nebulæ.
+ In considering the evolution of the stars let us keep two points clearly
+ in mind. The starting-point, the nebula, is no figment of the scientific
+ imagination. Hundreds of thousands of nebulæ, besides even vaster
+ irregular stretches of nebulous matter, exist in the heavens. But the
+ stages of the evolution of this stuff into stars are very largely a
+ matter of speculation. Possibly there is more than one line of
+ evolution, and the various theories may be reconciled. And this applies
+ also to the theories of the various stages through which the stars
+ themselves pass on their way to extinction.
+ The light of about a quarter of a million stars has been analysed in the
+ spectroscope, and it is found that they fall into about a dozen classes
+ which generally correspond to stages in their evolution (Fig. 21).
+ The Age of Stars
+ In its main lines the spectrum of a star corresponds to its colour, and
+ we may roughly group the stars into red, yellow, and white. This is also
+ the order of increasing temperature, the red stars being the coolest and
+ the white stars the hottest. We might therefore imagine that the white
+ stars are the youngest, and that as they grow older and cooler they
+ become yellowish, then red, and finally become invisible--just as a
+ cooling white-hot iron would do. But a very interesting recent research
+ shows that there are two kinds of red stars; some of them are amongst
+ the oldest stars and some are amongst the youngest. The facts appear to
+ be that when a star is first formed it is not very hot. It is an immense
+ mass of diffuse gas glowing with a dull-red heat. It contracts under the
+ mutual gravitation of its particles, and as it does so it grows hotter.
+ It acquires a yellowish tinge. As it continues to contract it grows
+ hotter and hotter until its temperature reaches a maximum as a white
+ star. At this point the contraction process does not stop, but the
+ heating process does. Further contraction is now accompanied by cooling,
+ and the star goes through its colour changes again, but this time in the
+ inverse order. It contracts and cools to yellow and finally to red. But
+ when it again becomes a red star it is enormously denser and smaller
+ than when it began as a red star. Consequently the red stars are divided
+ into two classes called, appropriately, Giants and Dwarfs. This theory,
+ which we owe to an American astronomer, H. N. Russell, has been
+ successful in explaining a variety of phenomena, and there is
+ consequently good reason to suppose it to be true. But the question as
+ to how the red giant stars were formed has received less satisfactory
+ and precise answers.
+ The most commonly accepted theory is the nebular theory.
+ THE NEBULAR THEORY
+ Nebulæ are dim luminous cloud-like patches in the heavens, more like
+ wisps of smoke in some cases than anything else. Both photography and
+ the telescope show that they are very numerous, hundreds of thousands
+ being already known and the number being continually added to. They are
+ not small. Most of them are immensely large. Actual dimensions cannot be
+ given, because to estimate these we must first know definitely the
+ distance of the nebulæ from the earth. The distances of some nebulæ are
+ known approximately, and we can therefore form some idea of size in
+ these cases. The results are staggering. The mere visible surface of
+ some nebulæ is so large that the whole stretch of the solar system would
+ be too small to form a convenient unit for measuring it. A ray of light
+ would require to travel for years to cross from side to side of such a
+ nebula. Its immensity is inconceivable to the human mind.
+ There appear to be two types of nebulæ, and there is evidence suggesting
+ that the one type is only an earlier form of the other; but this again
+ we do not know.
+ The more primitive nebulæ would seem to be composed of gas in an
+ extremely rarified form. It is difficult to convey an adequate idea of
+ the rarity of nebular gases. The residual gases in a vacuum tube are
+ dense by comparison. A cubic inch of air at ordinary pressure would
+ contain more matter than is contained in millions of cubic inches of the
+ gases of nebulæ. The light of even the faintest stars does not seem to
+ be dimmed by passing through a gaseous nebula, although we cannot be
+ sure on this point. The most remarkable physical fact about these gases
+ is that they are luminous. Whence they derive their luminosity we do not
+ know. It hardly seems possible to believe that extremely thin gases
+ exposed to the terrific cold of space can be so hot as to be luminous
+ and can retain their heat and their luminosity indefinitely. A cold
+ luminosity due to electrification, like that of the aurora borealis,
+ would seem to fit the case better.
+ Now the nebular theory is that out of great "fire-mists," such as we
+ have described, stars are born. We do not know whether gravitation is
+ the only or even the main force at work in a nebula, but it is supposed
+ that under the action of gravity the far-flung "fire-mists" would begin
+ to condense round centres of greatest density, heat being evolved in the
+ process. Of course the condensation would be enormously slow, although
+ the sudden irruption of a swarm of meteors or some solid body might
+ hasten matters greatly by providing large, ready-made centres of
+ condensation.
+ Spiral Nebulæ
+ It is then supposed that the contracting mass of gas would begin to
+ rotate and to throw off gigantic streamers, which would in their turn
+ form centres of condensation. The whole structure would thus form a
+ spiral, having a dense region at its centre and knots or lumps of
+ condensed matter along its spiral arms. Besides the formless gaseous
+ nebulæ there are hundreds of thousands of "spiral" nebulæ such as we
+ have just mentioned in the heavens. They are at all stages of
+ development, and they are visible to us at all angles--that is to say,
+ some of them face directly towards us, others are edge on, and some are
+ in intermediate positions. It appears, therefore, that we have here a
+ striking confirmation of the nebular hypothesis. But we must not go so
+ fast. There is much controversy as to the nature of these spiral nebulæ.
+ Some eminent astronomers think they are other stellar universes,
+ comparable in size with our own. In any case they are vast structures,
+ and if they represent stars in process of condensation, they must be
+ giving birth to huge agglomerations of stars--to star clusters at least.
+ These vast and enigmatic objects do not throw much light on the origin
+ of our own solar system. The nebular hypothesis, which was invented
+ by Laplace to explain the origin of our solar system, has not yet met
+ with universal acceptance. The explanation offers grave difficulties,
+ and it is best while the subject is still being closely investigated, to
+ hold all opinions with reserve. It may be taken as probable, however,
+ that the universe has developed from masses of incandescent gas.
+ [Illustration: _Photo: Yerkes Observatory._
+ FIG. 24.--THE GREAT NEBULA IN ORION
+ The most impressive nebula in the heavens. It is inconceivably greater
+ in dimensions than the whole solar system.]
+ [Illustration: _Photo: Lick Observatory._
+ FIG. 25--GIANT SPIRAL NEBULA, March 23, 1914
+ This spiral nebula is seen full on. Notice the central nucleus and the
+ two spiral arms emerging from its opposite directions. Is matter flowing
+ out of the nucleus into the arms or along the arms into the nucleus? In
+ either case we should get two streams in opposite directions within the
+ nucleus.]
+ THE BIRTH AND DEATH OF STARS
+ Variable, New, and Dark Stars: Dying Suns
+ Many astronomers believe that in "variable stars" we have another star,
+ following that of the dullest red star, in the dying of suns. The light
+ of these stars varies periodically in so many days, weeks, or years. It
+ is interesting to speculate that they are slowly dying suns, in which
+ the molten interior periodically bursts through the shell of thick
+ vapours that is gathering round them. What we saw about our sun seems to
+ point to some such stage in the future. That is, however, not the
+ received opinion about variable stars. It may be that they are stars
+ which periodically pass through a great swarm of meteors or a region of
+ space that is rich in cosmic dust of some sort, when, of course, a great
+ illumination would take place.
+ One class of these variable stars, which takes its name from the star
+ Algol, is of special interest. Every third night Algol has its light
+ reduced for several hours. Modern astronomy has discovered that in this
+ case there are really two stars, circulating round a common centre, and
+ that every third night the fainter of the two comes directly between us
+ and its companion and causes an "eclipse." This was until recently
+ regarded as a most interesting case in which a dead star revealed itself
+ to us by passing before the light of another star. But astronomers have
+ in recent years invented something, the "selenium-cell," which is even
+ more sensitive than the photographic plate, and on this the supposed
+ dead star registers itself as very much alive. Algol is, however,
+ interesting in another way. The pair of stars which we have discovered
+ in it are hundreds of trillions of miles away from the earth, yet we
+ know their masses and their distances from each other.
+ The Death and Birth of Stars
+ We have no positive knowledge of dead stars; which is not surprising
+ when we reflect that a dead star means an invisible star! But when we
+ see so many individual stars tending toward death, when we behold a vast
+ population of all conceivable ages, we presume that there are many
+ already dead. On the other hand, there is no reason to suppose that the
+ universe as a whole is "running down." Some writers have maintained
+ this, but their argument implies that we know a great deal more about
+ the universe than we actually do. The scientific man does not know
+ whether the universe is finite or infinite, temporal or eternal; and he
+ declines to speculate where there are no facts to guide him. He knows
+ only that the great gaseous nebulæ promise myriads of worlds in the
+ future, and he concedes the possibility that new nebulæ may be forming
+ in the ether of space.
+ The last, and not the least interesting, subject we have to notice is
+ the birth of a "new star." This is an event which astronomers now
+ announce every few years; and it is a far more portentous event than the
+ reader imagines when it is reported in his daily paper. The story is
+ much the same in all cases. We say that the star appeared in 1901, but
+ you begin to realise the magnitude of the event when you learn that the
+ distant "blaze" had really occurred about the time of the death of
+ Luther! The light of the conflagration had been speeding toward us
+ across space at 186,000 miles a second, yet it has taken nearly three
+ centuries to reach us. To be visible at all to us at that distance the
+ fiery outbreak must have been stupendous. If a mass of petroleum ten
+ times the size of the earth were suddenly fired it would not be seen at
+ such a distance. The new star had increased its light many hundredfold
+ in a few days.
+ There is a considerable fascination about the speculation that in such
+ cases we see the resurrection of a dead world, a means of renewing the
+ population of the universe. What happens is that in some region of the
+ sky where no star, or only a very faint star, had been registered on our
+ charts, we almost suddenly perceive a bright star. In a few days it may
+ rise to the highest brilliancy. By the spectroscope we learn that this
+ distant blaze means a prodigious outpour of white-hot hydrogen at
+ hundreds of miles a second. But the star sinks again after a few months,
+ and we then find a nebula round it on every side. It is natural to
+ suppose that a dead or dying sun has somehow been reconverted in whole
+ or in part into a nebula. A few astronomers think that it may have
+ partially collided with another star, or approached too closely to
+ another, with the result we described on an earlier page. The general
+ opinion now is that a faint or dead star had rushed into one of those
+ regions of space in which there are immense stretches of nebulous
+ matter, and been (at least in part) vaporised by the friction.
+ But the difficulties are considerable, and some astronomers prefer to
+ think that the blazing star may merely have lit up a dark nebula which
+ already existed. It is one of those problems on which speculation is
+ most tempting but positive knowledge is still very incomplete. We may be
+ content, even proud, that already we can take a conflagration that has
+ occurred more than a thousand trillion miles away and analyse it
+ positively into an outflame of glowing hydrogen gas at so many miles a
+ second.
+ THE SHAPE OF OUR UNIVERSE
+ Our Universe a Spiral Nebula
+ What is the shape of our universe, and what are its dimensions? This is
+ a tremendous question to ask. It is like asking an intelligent insect,
+ living on a single leaf in the midst of a great Brazilian forest, to say
+ what is the shape and size of the forest. Yet man's ingenuity has proved
+ equal to giving an answer even to this question, and by a method exactly
+ similar to that which would be adopted by the insect. Suppose, for
+ instance, that the forest was shaped as an elongated oval, and the
+ insect lived on a tree near the centre of the oval. If the trees were
+ approximately equally spaced from one another they would appear much
+ denser along the length of the oval than across its width. This is the
+ simple consideration that has guided astronomers in determining the
+ shape of our stellar universe. There is one direction in the heavens
+ along which the stars appear denser than in the directions at right
+ angles to it. That direction is the direction in which we look towards
+ the Milky Way. If we count the number of stars visible all over the
+ heavens, we find they become more and more numerous as we approach the
+ Milky Way. As we go farther and farther from the Milky Way the stars
+ thin out until they reach a maximum sparseness in directions at right
+ angles to the plane of the Milky Way. We may consider the Milky Way to
+ form, as it were, the equator of our system, and the line at right
+ angles to point to the north and south poles.
+ Our system, in fact, is shaped something like a lens, and our sun is
+ situated near the centre of this lens. In the remoter part of this lens,
+ near its edge, or possibly outside it altogether, lies the great series
+ of star clouds which make up the Milky Way. All the stars are in motion
+ within this system, but the very remarkable discovery has been made that
+ these motions are not entirely random. The great majority of the stars
+ whose motions can be measured fall into two groups drifting past one
+ another in opposite directions. The velocity of one stream relative to
+ the other is about twenty-five miles per second. The stars forming these
+ two groups are thoroughly well mixed; it is not a case of an inner
+ stream going one way and an outer stream the other. But there are not
+ quite as many stars going one way as the other. For every two stars in
+ one stream there are three in the other. Now, as we have said, some
+ eminent astronomers hold that the spiral nebulæ are universes like our
+ own, and if we look at the two photographs (Figs. 25 and 26) we see that
+ these spirals present features which, in the light of what we have just
+ said about our system, are very remarkable. The nebula in Coma Berenices
+ is a spiral edge-on to us, and we see that it has precisely the
+ lens-shaped middle and the general flattened shape that we have found in
+ our own system. The nebula in Canes Venatici is a spiral facing towards
+ us, and its shape irresistibly suggests motions along the spiral arms.
+ This motion, whether it is towards or away from the central, lens-shaped
+ portion, would cause a double streaming motion in that central portion
+ of the kind we have found in our own system. Again, and altogether apart
+ from these considerations, there are good reasons for supposing our
+ Milky Way to possess a double-armed spiral structure. And the great
+ patches of dark absorbing matter which are known to exist in the Milky
+ Way (see Fig. 22) would give very much the mottled appearance we notice
+ in the arms (which we see edge-on) of the nebula in Coma Berenices. The
+ hypothesis, therefore, that our universe is a spiral nebula has much to
+ be said for it. If it be accepted it greatly increases our estimate of
+ the size of the material universe. For our central, lens-shaped system
+ is calculated to extend towards the Milky Way for more than twenty
+ thousand times a million million miles, and about a third of this
+ distance towards what we have called the poles. If, as we suppose, each
+ spiral nebula is an independent stellar universe comparable in size with
+ our own, then, since there are hundreds of thousands of spiral nebulæ,
+ we see that the size of the whole material universe is indeed beyond our
+ comprehension.
+ Notice the lens-shaped formation of the nucleus and the arm stretching
+ as a band across it. See reference in the text to the resemblance
+ between this and our stellar universe.]
+ 100-INCH TELESCOPE, MOUNT WILSON
+ A reflecting telescope: the largest in the world. The mirror is situated
+ at the base of the telescope.]
+ The above distances are merely approximate and are subject to further
+ revision. A "light-year" is the distance that light, travelling at the
+ rate of 186,000 miles per second, would cover in one year.]
+ In this simple outline we have not touched on some of the more debatable
+ questions that engage the attention of modern astronomers. Many of these
+ questions have not yet passed the controversial stage; out of these will
+ emerge the astronomy of the future. But we have seen enough to convince
+ us that, whatever advances the future holds in store, the science of the
+ heavens constitutes one of the most important stones in the wonderful
+ fabric of human knowledge.
+ ASTRONOMICAL INSTRUMENTS
+ The Telescope
+ The instruments used in modern astronomy are amongst the finest triumphs
+ of mechanical skill in the world. In a great modern observatory the
+ different instruments are to be counted by the score, but there are two
+ which stand out pre-eminent as the fundamental instruments of modern
+ astronomy. These instruments are the telescope and the spectroscope, and
+ without them astronomy, as we know it, could not exist.
+ There is still some dispute as to where and when the first telescope was
+ constructed; as an astronomical instrument, however, it dates from the
+ time of the great Italian scientist Galileo, who, with a very small and
+ imperfect telescope of his own invention, first observed the spots on
+ the sun, the mountains of the moon, and the chief four satellites of
+ Jupiter. A good pair of modern binoculars is superior to this early
+ instrument of Galileo's, and the history of telescope construction, from
+ that primitive instrument to the modern giant recently erected on Mount
+ Wilson, California, is an exciting chapter in human progress. But the
+ early instruments have only an historic interest: the era of modern
+ telescopes begins in the nineteenth century.
+ During the last century telescope construction underwent an
+ unprecedented development. An immense amount of interest was taken in
+ the construction of large telescopes, and the different countries of the
+ world entered on an exciting race to produce the most powerful possible
+ instruments. Besides this rivalry of different countries there was a
+ rivalry of methods. The telescope developed along two different lines,
+ and each of these two types has its partisans at the present day. These
+ types are known as _refractors_ and _reflectors_, and it is necessary to
+ mention, briefly, the principles employed in each. The _refractor_ is
+ the ordinary, familiar type of telescope. It consists, essentially, of a
+ large lens at one end of a tube, and a small lens, called the eye-piece,
+ at the other. The function of the large lens is to act as a sort of
+ gigantic eye. It collects a large amount of light, an amount
+ proportional to its size, and brings this light to a focus within the
+ tube of the telescope. It thus produces a small but bright image, and
+ the eye-piece magnifies this image. In the _reflector_, instead of a
+ large lens at the top of the tube, a large mirror is placed at the
+ bottom. This mirror is so shaped as to reflect the light that falls on
+ it to a focus, whence the light is again led to an eye-piece. Thus the
+ refractor and the reflector differ chiefly in their manner of gathering
+ light. The powerfulness of the telescope depends on the size of the
+ light-gatherer. A telescope with a lens four inches in diameter is four
+ times as powerful as the one with a lens two inches in diameter, for the
+ amount of light gathered obviously depends on the _area_ of the lens,
+ and the area varies as the _square_ of the diameter.
+ The largest telescopes at present in existence are _reflectors_. It is
+ much easier to construct a very large mirror than to construct a very
+ large lens; it is also cheaper. A mirror is more likely to get out of
+ order than is a lens, however, and any irregularity in the shape of a
+ mirror produces a greater distorting effect than in a lens. A refractor
+ is also more convenient to handle than is a reflector. For these reasons
+ great refractors are still made, but the largest of them, the great
+ Yerkes' refractor, is much smaller than the greatest reflector, the one
+ on Mount Wilson, California. The lens of the Yerkes' refractor measures
+ three feet four inches in diameter, whereas the Mount Wilson reflector
+ has a diameter of no less than eight feet four inches.
+ [Illustration: THE YERKES 40-INCH REFRACTOR
+ (The largest _refracting_ telescope in the world. Its big lens weighs
+ 1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about
+ 12,000 pounds. The parts to be moved weigh approximately 22 tons.
+ The great _100-inch reflector_ of the Mount Wilson reflecting
+ telescope--the largest _reflecting_ instrument in the world--weighs
+ nearly 9,000 pounds and the moving parts of the telescope weigh about
+ 100 tons.
+ The new _72-inch reflector_ at the Dominion Astrophysical Observatory,
+ near Victoria, B. C., weighs nearly 4,500 pounds, and the moving parts
+ about 35 tons.)]
+ [Illustration: _Photo: H. J. Shepstone._
+ THE DOUBLE-SLIDE PLATE HOLDER ON YERKES 40-INCH REFRACTING TELESCOPE
+ The smaller telescope at the top of the picture acts as a "finder"; the
+ field of view of the large telescope is so restricted that it is
+ difficult to recognise, as it were, the part of the heavens being
+ surveyed. The smaller telescope takes in a larger area and enables the
+ precise object to be examined to be easily selected.]
+ [Illustration: MODERN DIRECT-READING SPECTROSCOPE
+(_By A. Hilger, Ltd._)
+ The light is brought through one telescope, is split up by the prism,
+ and the resulting spectrum is observed through the other telescope.]
+ But there is a device whereby the power of these giant instruments,
+ great as it is, can be still further heightened. That device is the
+ simple one of allowing the photographic plate to take the place of the
+ human eye. Nowadays an astronomer seldom spends the night with his eye
+ glued to the great telescope. He puts a photographic plate there. The
+ photographic plate has this advantage over the eye, that it builds up
+ impressions. However long we stare at an object too faint to be seen, we
+ shall never see it. With the photographic plate, however, faint
+ impressions go on accumulating. As hour after hour passes, the star
+ which was too faint to make a perceptible impression on the plate goes
+ on affecting it until finally it makes an impression which can be made
+ visible. In this way the photographic plate reveals to us phenomena in
+ the heavens which cannot be seen even through the most powerful
+ telescopes.
+ Telescopes of the kind we have been discussing, telescopes for exploring
+ the heavens, are mounted _equatorially_; that is to say, they are
+ mounted on an inclined pillar parallel to the axis of the earth so that,
+ by rotating round this pillar, the telescope is enabled to follow the
+ apparent motion of a star due to the rotation of the earth. This motion
+ is effected by clock-work, so that, once adjusted on a star, and the
+ clock-work started, the telescope remains adjusted on that star for any
+ length of time that is desired. But a great official observatory, such
+ as Greenwich Observatory or the Observatory at Paris, also has _transit_
+ instruments, or telescopes smaller than the equatorials and without the
+ same facility of movement, but which, by a number of exquisite
+ refinements, are more adapted to accurate measurements. It is these
+ instruments which are chiefly used in the compilation of the _Nautical
+ Almanac_. They do not follow the apparent motions of the stars. Stars
+ are allowed to drift across the field of vision, and as each star
+ crosses a small group of parallel wires in the eye-piece its precise
+ time of passage is recorded. Owing to their relative fixity of position
+ these instruments can be constructed to record the _positions_ of stars
+ with much greater accuracy than is possible to the more general and
+ flexible mounting of equatorials. The recording of transit is
+ comparatively dry work; the spectacular element is entirely absent;
+ stars are treated merely as mathematical points. But these observations
+ furnish the very basis of modern mathematical astronomy, and without
+ them such publications as the _Nautical Almanac_ and the _Connaissance
+ du Temps_ would be robbed of the greater part of their importance.
+ The Spectroscope
+ We have already learnt something of the principles of the spectroscope,
+ the instrument which, by making it possible to learn the actual
+ constitution of the stars, has added a vast new domain to astronomy. In
+ the simplest form of this instrument the analysing portion consists of a
+ single prism. Unless the prism is very large, however, only a small
+ degree of dispersion is obtained. It is obviously desirable, for
+ accurate analytical work, that the dispersion--that is, the separation
+ of the different parts of the spectrum--should be as great as possible.
+ The dispersion can be increased by using a large number of prisms, the
+ light emerging from the first prism, entering the second, and so on. In
+ this way each prism produces its own dispersive effect and, when a
+ number of prisms are employed, the final dispersion is considerable. A
+ considerable amount of light is absorbed in this way, however, so that
+ unless our primary source of light is very strong, the final spectrum
+ will be very feeble and hard to decipher.
+ Another way of obtaining considerable dispersion is by using a
+ _diffraction grating_ instead of a prism. This consists essentially of a
+ piece of glass on which lines are ruled by a diamond point. When the
+ lines are sufficiently close together they split up light falling on
+ them into its constituents and produce a spectrum. The modern
+ diffraction grating is a truly wonderful piece of work. It contains
+ several thousands of lines to the inch, and these lines have to be
+ spaced with the greatest accuracy. But in this instrument, again, there
+ is a considerable loss of light.
+ We have said that every substance has its own distinctive spectrum, and
+ it might be thought that, when a list of the spectra of different
+ substances has been prepared, spectrum analysis would become perfectly
+ straightforward. In practice, however, things are not quite so simple.
+ The spectrum emitted by a substance is influenced by a variety of
+ conditions. The pressure, the temperature, the state of motion of the
+ object we are observing, all make a difference, and one of the most
+ laborious tasks of the modern spectroscopist is to disentangle these
+ effects from one another. Simple as it is in its broad outlines,
+ spectroscopy is, in reality, one of the most intricate branches of
+ modern science.
+ BIBLIOGRAPHY
+ (The following list of books may be useful to readers wishing to pursue
+ further the study of Astronomy.)
+ BALL, _The Story of the Heavens_.
+ BALL, _The Story of the Sun_.
+ FORBES, _History of Astronomy_.
+ HINCKS, _Astronomy_.
+ KIPPAX, _Call of the Stars_.
+ LOWELL, _Mars and Its Canals_.
+ LOWELL, _Evolution of Worlds_.
+ MCKREADY, _A Beginner's Star-Book_.
+ NEWCOMB, _Popular Astronomy_.
+ NEWCOMB, _The Stars: A Study of the Universe_.
+ OLCOTT, _Field Book of the Stars_.
+ PRICE, _Essence of Astronomy_.
+ SERVISS, _Curiosities of the Skies_.
+ WEBB, _Celestial Objects for Common Telescopes_.
+ YOUNG, _Text-Book of General Astronomy_.
+ II
+ THE STORY OF EVOLUTION
+ INTRODUCTORY
+ THE BEGINNING OF THE EARTH--MAKING A HOME FOR LIFE--THE FIRST LIVING
+ CREATURES
+ The Evolution-idea is a master-key that opens many doors. It is a
+ luminous interpretation of the world, throwing the light of the past
+ upon the present. Everything is seen to be an antiquity, with a history
+ behind it--a _natural history_, which enables us to understand in some
+ measure how it has come to be as it is. We cannot say more than
+ "understand in some measure," for while the _fact_ of evolution is
+ certain, we are only beginning to discern the _factors_ that have been
+ at work.
+ The evolution-idea is very old, going back to some of the Greek
+ philosophers, but it is only in modern times that it has become an
+ essential part of our mental equipment. It is now an everyday
+ intellectual tool. It was applied to the origin of the solar system and
+ to the making of the earth before it was applied to plants and animals;
+ it was extended from these to man himself; it spread to language, to
+ folk-ways, to institutions. Within recent years the evolution-idea has
+ been applied to the chemical elements, for it appears that uranium may
+ change into radium, that radium may produce helium, and that lead is the
+ final stable result when the changes of uranium are complete. Perhaps
+ all the elements may be the outcome of an inorganic evolution. Not less
+ important is the extension of the evolution-idea to the world within as
+ well as to the world without. For alongside of the evolution of bodies
+ and brains is the evolution of feelings and emotions, ideas and
+ imagination.
+ Organic evolution means that the present is the child of the past and
+ the parent of the future. It is not a power or a principle; it is a
+ process--a process of becoming. It means that the present-day animals
+ and plants and all the subtle inter-relations between them have arisen
+ in a natural knowable way from a preceding state of affairs on the whole
+ somewhat simpler, and that again from forms and inter-relations simpler
+ still, and so on backwards and backwards for millions of years till we
+ lose all clues in the thick mist that hangs over life's beginnings.
+ Our solar system was once represented by a nebula of some sort, and we
+ may speak of the evolution of the sun and the planets. But since it has
+ been _the same material throughout_ that has changed in its distribution
+ and forms, it might be clearer to use some word like genesis. Similarly,
+ our human institutions were once very different from what they are now,
+ and we may speak of the evolution of government or of cities. But Man
+ works with a purpose, with ideas and ideals in some measure controlling
+ his actions and guiding his achievements, so that it is probably clearer
+ to keep the good old word history for all processes of social becoming
+ in which man has been a conscious agent. Now between the genesis of the
+ solar system and the history of civilisation there comes the vast
+ process of organic evolution. The word development should be kept for
+ the becoming of the individual, the chick out of the egg, for instance.
+ Organic evolution is a continuous natural process of racial change, by
+ successive steps in a definite direction, whereby distinctively new
+ individualities arise, take root, and flourish, sometimes alongside of,
+ and sometimes, sooner or later, in place of, the originative stock. Our
+ domesticated breeds of pigeons and poultry are the results of
+ evolutionary change whose origins are still with us in the Rock Dove and
+ the Jungle Fowl; but in most cases in Wild Nature the ancestral stocks
+ of present-day forms are long since extinct, and in many cases they are
+ unknown. Evolution is a long process of coming and going, appearing and
+ disappearing, a long-drawn-out sublime process like a great piece of
+ music.
+ [Illustration: _Photo: Rischgitz Collection._
+ CHARLES DARWIN
+ Greatest of naturalists, who made the idea of evolution current
+ intellectual coin, and in his _Origin of Species_ (1859) made the whole
+ world new.]
+ [Illustration: _Photo: Rischgitz Collection._
+ LORD KELVIN
+ One of the greatest physicists of the nineteenth century. He estimated
+ the age of the earth at 20,000,000 years. He had not at his disposal,
+ however, the knowledge of recent discoveries, which have resulted in
+ this estimate being very greatly increased.]
+ [Illustration: _Photo: Lick Observatory._
+ A GIANT SPIRAL NEBULA
+ Laplace's famous theory was that the planets and the earth were formed
+ from great whirling nebulæ.]
+ [Illustration: _Photo: Natural History Museum._
+ METEORITE WHICH FELL NEAR SCARBOROUGH, AND IS NOW TO BE SEEN IN THE
+ NATURAL HISTORY MUSEUM
+ It weighs about 56 lb., and is a "stony" meteorite, i.e., an aerolite.]
+ The Beginning of the Earth
+ When we speak the language of science we cannot say "In the beginning,"
+ for we do not know of and cannot think of any condition of things that
+ did not arise from something that went before. But we may qualify the
+ phrase, and legitimately inquire into the beginning of the earth within
+ the solar system. If the result of this inquiry is to trace the sun and
+ the planets back to a nebula we reach only a relative beginning. The
+ nebula has to be accounted for. And even before matter there may have
+ been a pre-material world. If we say, as was said long ago, "In the
+ beginning was Mind," we may be expressing or trying to express a great
+ truth, but we have gone BEYOND SCIENCE.
+ The Nebular Hypothesis
+ One of the grandest pictures that the scientific mind has ever thrown
+ upon the screen is that of the Nebular Hypothesis. According to
+ Laplace's famous form of this theory (1796), the solar system was once a
+ gigantic glowing mass, spinning slowly and uniformly around its centre.
+ As the incandescent world-cloud of gas cooled and its speed of rotation
+ increased the shrinking mass gave off a separate whirling ring, which
+ broke up and gathered together again as the first and most distant
+ planet. The main mass gave off another ring and another till all the
+ planets, including the earth, were formed. The central mass persisted as
+ the sun.
+ Laplace spoke of his theory, which Kant had anticipated forty-one years
+ before, with scientific caution: "conjectures which I present with all
+ the distrust which everything not the result of observation or of
+ calculation ought to inspire." Subsequent research justified his
+ distrust, for it has been shown that the original nebula need not have
+ been hot and need not have been gaseous. Moreover, there are great
+ difficulties in Laplace's theory of the separation of successive rings
+ from the main mass, and of the condensation of a whirling gaseous ring
+ into a planet.
+ So it has come about that the picture of a hot gaseous nebula revolving
+ as a unit body has given place to other pictures. Thus Sir Norman
+ Lockyer pointed out (1890) that the earth is gathering to itself
+ millions of meteorites every day; this has been going on for millions of
+ years; in distant ages the accretion may have been vastly more rapid and
+ voluminous; and so the earth has grown! Now the meteoritic contributions
+ are undoubted, but they require a centre to attract them, and the
+ difficulty is to account for the beginning of a collecting centre or
+ planetary nucleus. Moreover, meteorites are sporadic and erratic,
+ scattered hither and thither rather than collecting into unit-bodies. As
+ Professor Chamberlin says, "meteorites have rather the characteristics
+ of the wreckage of some earlier organisation than of the parentage of
+ our planetary system." Several other theories have been propounded to
+ account for the origin of the earth, but the one that has found most
+ favour in the eyes of authorities is that of Chamberlin and Moulton.
+ According to this theory a great nebular mass condensed to form the sun,
+ from which under the attraction of passing stars planet after planet,
+ the earth included, was heaved off in the form of knotted spiral nebulæ,
+ like many of those now observed in the heavens.
+ Of great importance were the "knots," for they served as collecting
+ centres drawing flying matter into their clutches. Whatever part of the
+ primitive bolt escaped and scattered was drawn out into independent
+ orbits round the sun, forming the "planetesimals" which behave like
+ minute planets. These planetesimals formed the food on which the knots
+ subsequently fed.
+ The Growth of the Earth
+ It has been calculated that the newborn earth--the "earth-knot" of
+ Chamberlin's theory--had a diameter of about 5,500 miles. But it grew
+ by drawing planetesimals into itself until it had a diameter of over
+ 8,100 miles at the end of its growing period. Since then it has shrunk,
+ by periodic shrinkages which have meant the buckling up of successive
+ series of mountains, and it has now a diameter of 7,918 miles. But
+ during the shrinking the earth became more varied.
+ A sort of slow boiling of the internally hot earth often forced molten
+ matter through the cold outer crust, and there came about a gradual
+ assortment of lighter materials nearer the surface and heavier materials
+ deeper down. The continents are built of the lighter materials, such as
+ granites, while the beds of the great oceans are made of the heavier
+ materials such as basalts. In limited areas land has often become sea,
+ and sea has often given place to land, but the probability is that the
+ distinction of the areas corresponding to the great continents and
+ oceans goes back to a very early stage.
+ The lithosphere is the more or less stable crust of the earth, which may
+ have been, to begin with, about fifty miles in thickness. It seems that
+ the young earth had no atmosphere, and that ages passed before water
+ began to accumulate on its surface--before, in other words, there was
+ any hydrosphere. The water came from the earth itself, to begin with,
+ and it was long before there was any rain dissolving out saline matter
+ from the exposed rocks and making the sea salt. The weathering of the
+ high grounds of the ancient crust by air and water furnished the
+ material which formed the sandstones and mudstones and other sedimentary
+ rocks, which are said to amount to a thickness of over fifty miles in
+ all.
+ Making a Home for Life
+ It is interesting to inquire how the callous, rough-and-tumble
+ conditions of the outer world in early days were replaced by others that
+ allowed of the germination and growth of that tender plant we call
+ LIFE. There are very tough living creatures, but the average organism is
+ ill suited for violence. Most living creatures are adapted to mild
+ temperatures and gentle reactions. Hence the fundamental importance of
+ the early atmosphere, heavy with planetesimal dust, in blanketing the
+ earth against intensities of radiance from without, as Chamberlin says,
+ and inequalities of radiance from within. This was the first preparation
+ for life, but it was an atmosphere without free oxygen. Not less
+ important was the appearance of pools and lakelets, of lakes and seas.
+ Perhaps the early waters covered the earth. And water was the second
+ preparation for life--water, that can dissolve a larger variety of
+ substances in greater concentration than any other liquid; water, that
+ in summer does not readily evaporate altogether from a pond, nor in
+ winter freeze throughout its whole extent; water, that is such a mobile
+ vehicle and such a subtle cleaver of substances; water, that forms over
+ 80 per cent. of living matter itself.
+ Of great significance was the abundance of carbon, hydrogen, and oxygen
+ (in the form of carbonic acid and water) in the atmosphere of the
+ cooling earth, for these three wonderful elements have a unique
+ _ensemble_ of properties--ready to enter into reactions and relations,
+ making great diversity and complexity possible, favouring the formation
+ of the plastic and permeable materials that build up living creatures.
+ We must not pursue the idea, but it is clear that the stones and mortar
+ of the inanimate world are such that they built a friendly home for
+ life.
+ Origin of Living Creatures upon the Earth
+ During the early chapters of the earth's history, no living creature
+ that we can imagine could possibly have lived there. The temperature was
+ too high; there was neither atmosphere nor surface water. Therefore it
+ follows that at some uncertain, but inconceivably distant date, living
+ creatures appeared upon the earth. No one knows how, but it is
+ interesting to consider possibilities.
+ [Illustration: _Reproduced from the Smithsonian Report, 1915._
+ A LIMESTONE CANYON
+ Many fossils of extinct animals have been found in such rock
+ formations.]
+ [Illustration: GENEALOGICAL TREE OF ANIMALS
+ Showing in order of evolution the general relations of the chief classes
+ into which the world of living things is divided. This scheme represents
+ the present stage of our knowledge, but is admittedly provisional.]
+ [Illustration: DIAGRAM OF AMOEBA
+(Greatly magnified.)
+ The amoeba is one of the simplest of all animals, and gives us a hint
+ of the original ancestors. It looks like a tiny irregular speck of
+ greyish jelly, about 1/100th of an inch in diameter. It is commonly
+ found gliding on the mud or weeds in ponds, where it engulfs its
+ microscopic food by means of out-flowing lobes (PS). The food vacuole
+ (FV) contains ingested food. From the contractile vacuole (CV) the waste
+ matter is discharged. N is the nucleus, GR, granules.]
+ From ancient times it has been a favourite answer that the dust of the
+ earth may have become living in a way which is outside scientific
+ description. This answer forecloses the question, and it is far too soon
+ to do that. Science must often say "Ignoramus": Science should be slow
+ to say "Ignorabimus."
+ A second position held by Helmholtz, Lord Kelvin, and others, suggests
+ that minute living creatures may have come to the earth from elsewhere,
+ in the cracks of a meteorite or among cosmic dust. It must be remembered
+ that seeds can survive prolonged exposure to very low temperatures; that
+ spores of bacteria can survive high temperature; that seeds of plants
+ and germs of animals in a state of "latent life" can survive prolonged
+ drought and absence of oxygen. It is possible, according to Berthelot,
+ that as long as there is not molecular disintegration vital activities
+ may be suspended for a time, and may afterwards recommence when
+ appropriate conditions are restored. Therefore, one should be slow to
+ say that a long journey through space is impossible. The obvious
+ limitation of Lord Kelvin's theory is that it only shifts the problem of
+ the origin of organisms (i.e. living creatures) from the earth to
+ elsewhere.
+ The third answer is that living creatures of a very simple sort may have
+ emerged on the earth's surface from not-living material, e.g. from some
+ semi-fluid carbon compounds activated by ferments. The tenability of
+ this view is suggested by the achievements of the synthetic chemists,
+ who are able artificially to build up substances such as oxalic acid,
+ indigo, salicylic acid, caffeine, and grape-sugar. We do not know,
+ indeed, what in Nature's laboratory would take the place of the clever
+ synthetic chemist, but there seems to be a tendency to complexity.
+ Corpuscles form atoms, atoms form molecules, small molecules large
+ ones.
+ Various concrete suggestions have been made in regard to the possible
+ origin of living matter, which will be dealt with in a later chapter. So
+ far as we know of what goes on to-day, there is no evidence of
+ spontaneous generation; organisms seem always to arise from pre-existing
+ organisms of the same kind; where any suggestion of the contrary has
+ been fancied, there have been flaws in the experimenting. But it is one
+ thing to accept the verdict "omne vivum e vivo" as a fact to which
+ experiment has not yet discovered an exception and another thing to
+ maintain that this must always have been true or must always remain
+ true.
+ If the synthetic chemists should go on surpassing themselves, if
+ substances like white of egg should be made artificially, and if we
+ should get more light on possible steps by which simple living creatures
+ may have arisen from not-living materials, this would not greatly affect
+ our general outlook on life, though it would increase our appreciation
+ of what is often libelled as "inert" matter. If the dust of the earth
+ did naturally give rise very long ago to living creatures, if they are
+ in a real sense born of her and of the sunshine, then the whole world
+ becomes more continuous and more vital, and all the inorganic groaning
+ and travailing becomes more intelligible.
+ The First Organisms upon the Earth
+ We cannot have more than a speculative picture of the first living
+ creatures upon the earth or, rather, in the waters that covered the
+ earth. A basis for speculation is to be found, however, in the simplest
+ creatures living to-day, such as some of the bacteria and one-celled
+ animalcules, especially those called Protists, which have not taken any
+ very definite step towards becoming either plants or animals. No one can
+ be sure, but there is much to be said for the theory that the first
+ creatures were microscopic globules of living matter, not unlike the
+ simplest bacteria of to-day, but able to live on air, water, and
+ dissolved salts. From such a source may have originated a race of
+ one-celled marine organisms which were able to manufacture chlorophyll,
+ or something like chlorophyll, that is to say, the green pigment which
+ makes it possible for plants to utilise the energy of the sunlight in
+ breaking up carbon dioxide and in building up (photosynthesis) carbon
+ compounds like sugars and starch. These little units were probably
+ encased in a cell-wall of cellulose, but their boxed-in energy expressed
+ itself in the undulatory movement of a lash or flagellum, by means of
+ which they propelled themselves energetically through the water. There
+ are many similar organisms to-day, mostly in water, but some of
+ them--simple one-celled plants--paint the tree-stems and even the
+ paving-stones green in wet weather. According to Prof. A. H. Church
+ there was a long chapter in the history of the earth when the sea that
+ covered everything teemed with these green flagellates--the originators
+ of the Vegetable Kingdom.
+ On another tack, however, there probably evolved a series of simple
+ predatory creatures, not able to build up organic matter from air,
+ water, and salts, but devouring their neighbours. These units were not
+ closed in with cellulose, but remained naked, with their living matter
+ or protoplasm flowing out in changeful processes, such as we see in the
+ Amoebæ in the ditch or in our own white blood corpuscles and other
+ amoeboid cells. These were the originators of the animal kingdom. Thus
+ from very simple Protists the first animals and the first plants may
+ have arisen. All were still very minute, and it is worth remembering
+ that had there been any scientific spectator after our kind upon the
+ earth during these long ages, he would have lamented the entire absence
+ of life, although the seas were teeming. The simplest forms of life and
+ the protoplasm which Huxley called the physical basis of life will be
+ dealt with in the chapter on Biology in a later section of this work.
+ FIRST GREAT STEPS IN EVOLUTION
+ THE FIRST PLANTS--THE FIRST ANIMALS--BEGINNINGS OF BODIES--EVOLUTION OF
+ SEX--BEGINNING OF NATURAL DEATH
+ The Contrast between Plants and Animals
+ However it may have come about, there is no doubt at all that one of the
+ first great steps in Organic Evolution was the forking of the
+ genealogical tree into Plants and Animals--the most important parting of
+ the ways in the whole history of Nature.
+ Typical plants have chlorophyll; they are able to feed at a low chemical
+ level on air, water, and salts, using the energy of the sunlight in
+ their photosynthesis. They have their cells boxed in by cellulose walls,
+ so that their opportunities for motility are greatly restricted. They
+ manufacture much more nutritive material than they need, and live far
+ below their income. They have no ready way of getting rid of any
+ nitrogenous waste matter that they may form, and this probably helps to
+ keep them sluggish.
+ Animals, on the other hand, feed at a high chemical level, on the
+ carbohydrates (e.g. starch and sugar), fats, and proteins (e.g. gluten,
+ albumin, casein) which are manufactured by other animals, or to begin
+ with, by plants. Their cells have not cellulose walls, nor in most cases
+ much wall of any kind, and motility in the majority is unrestricted.
+ Animals live much more nearly up to their income. If we could make for
+ an animal and a plant of equal weight two fractions showing the ratio of
+ the upbuilding, constructive, chemical processes to the down-breaking,
+ disruptive, chemical processes that go on in their respective bodies,
+ the ratio for the plant would be much greater than the corresponding
+ ratio for the animal. In other words, animals take the munitions which
+ plants laboriously manufacture and explode them in locomotion and
+ work; and the entire system of animate nature depends upon the
+ photosynthesis that goes on in green plants.
+ [Illustration: _From the Smithsonian Report, 1917_
+ A PIECE OF A REEF-BUILDING CORAL, BUILT UP BY A LARGE COLONY OF SMALL
+ SEA-ANEMONE-LIKE POLYPS, EACH OF WHICH FORMS FROM THE SALTS OF THE SEA A
+ SKELETON OR SHELL OF LIME
+ The wonderful mass of corals, which are very beautiful, are the skeleton
+ remains of hundreds of these little creatures.]
+ [Illustration: _Photo: J. J. Ward, F.E.S._
+ THE INSET CIRCLE SHOWS A GROUP OF CHALK-FORMING ANIMALS, OR
+ FORAMINIFERA, EACH ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD
+ They form a great part of the chalk cliffs of Dover and similar deposits
+ which have been raised from the floor of an ancient sea.
+ THE ENORMOUSLY ENLARGED ILLUSTRATION IS THAT OF A COMMON FORAMINIFER
+ (POLYSTOMELLA) SHOWING THE SHELL IN THE CENTRE AND THE OUTFLOWING
+ NETWORK OF LIVING MATTER, ALONG WHICH GRANULES ARE CONTINUALLY
+ TRAVELLING, AND BY WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN
+ _Reproduced by permission of the Natural History Museum_ (_after Max
+ Schultze_).]
+ As the result of much more explosive life, animals have to deal with
+ much in the way of nitrogenous waste products, the ashes of the living
+ fire, but these are usually got rid of very effectively, e.g. in the
+ kidney filters, and do not clog the system by being deposited as
+ crystals and the like, as happens in plants. Sluggish animals like
+ sea-squirts which have no kidneys are exceptions that prove the rule,
+ and it need hardly be said that the statements that have been made in
+ regard to the contrasts between plants and animals are general
+ statements. There is often a good deal of the plant about the animal, as
+ in sedentary sponges, zoophytes, corals, and sea-squirts, and there is
+ often a little of the animal about the plant, as we see in the movements
+ of all shoots and roots and leaves, and occasionally in the parts of the
+ flower. But the important fact is that on the early forking of the
+ genealogical tree, i.e. the divergence of plants and animals, there
+ depended and depends all the higher life of the animal kingdom, not to
+ speak of mankind. The continuance of civilisation, the upkeep of the
+ human and animal population of the globe, and even the supply of oxygen
+ to the air we breathe, depend on the silent laboratories of the green
+ leaves, which are able with the help of the sunlight to use carbonic
+ acid, water, and salts to build up the bread of life.
+ The Beginnings of Land Plants
+ It is highly probable that for long ages the waters covered the earth,
+ and that all the primeval vegetation consisted of simple Flagellates in
+ the universal Open Sea. But contraction of the earth's crust brought
+ about elevations and depressions of the sea-floor, and in places the
+ solid substratum was brought near enough the surface to allow the
+ floating plants to begin to settle down without getting out of the
+ light. This is how Professor Church pictures the beginning of a fixed
+ vegetation--a very momentous step in evolution. It was perhaps among
+ this early vegetation that animals had their first successes. As the
+ floor of the sea in these shallow areas was raised higher and higher
+ there was a beginning of dry land. The sedentary plants already spoken
+ of were the ancestors of the shore seaweeds, and there is no doubt that
+ when we go down at the lowest tide and wade cautiously out among the
+ jungle of vegetation only exposed on such occasions we are getting a
+ glimpse of very ancient days. _This_ is the forest primeval.
+ The Protozoa
+ Animals below the level of zoophytes and sponges are called Protozoa.
+ The word obviously means "First Animals," but all that we can say is
+ that the very simplest of them may give us some hint of the simplicity
+ of the original first animals. For it is quite certain that the vast
+ majority of the Protozoa to-day are far too complicated to be thought of
+ as primitive. Though most of them are microscopic, each is an animal
+ complete in itself, with the same fundamental bodily attributes as are
+ manifested in ourselves. They differ from animals of higher degree in
+ not being built up of the unit areas or corpuscles called cells. They
+ have no cells, no tissues, no organs, in the ordinary acceptation of
+ these words, but many of them show a great complexity of internal
+ structure, far exceeding that of the ordinary cells that build up the
+ tissues of higher animals. They are complete living creatures which have
+ not gone in for body-making.
+ In the dim and distant past there was a time when the only animals were
+ of the nature of Protozoa, and it is safe to say that one of the great
+ steps in evolution was the establishment of three great types of
+ Protozoa: (_a_) Some were very active, the Infusorians, like the slipper
+ animalcule, the night-light (Noctiluca), which makes the seas
+ phosphorescent at night, and the deadly Trypanosome, which causes
+ Sleeping Sickness. (_b_) Others were very sluggish, the parasitic
+ Sporozoa, like the malaria organism which the mosquito introduces into
+ man's body. (_c_) Others were neither very active nor very passive, the
+ Rhizopods, with out-flowing processes of living matter. This amoeboid
+ line of evolution has been very successful; it is represented by the
+ Rhizopods, such as Amoebæ and the chalk-forming Foraminifera and the
+ exquisitely beautiful flint-shelled Radiolarians of the open sea. They
+ have their counterparts in the amoeboid cells of most multicellular
+ animals, such as the phagocytes which migrate about in the body,
+ engulfing and digesting intruding bacteria, serving as sappers and
+ miners when something has to be broken down and built up again, and
+ performing other useful offices.
+ The Making of a Body
+ The great naturalist Louis Agassiz once said that the biggest gulf in
+ Organic Nature was that between the unicellular and the multicellular
+ animals (Protozoa and Metazoa). But the gulf was bridged very long ago
+ when sponges, stinging animals, and simple worms were evolved, and
+ showed, for the first time, a "body." What would one not give to be able
+ to account for the making of a body, one of the great steps in
+ evolution! No one knows, but the problem is not altogether obscure.
+ When an ordinary Protozoon or one-celled animal divides into two or
+ more, which is its way of multiplying, the daughter-units thus formed
+ float apart and live independent lives. But there are a few Protozoa in
+ which the daughter-units are not quite separated off from one another,
+ but remain coherent. Thus Volvox, a beautiful green ball, found in some
+ canals and the like, is a colony of a thousand or even ten thousand
+ cells. It has almost formed a body! But in this "colony-making"
+ Protozoon, and in others like it, the component cells are all of one
+ kind, whereas in true multicellular animals there are different kinds
+ of cells, showing division of labour. There are some other Protozoa in
+ which the nucleus or kernel divides into many nuclei within the cell.
+ This is seen in the Giant Amoeba (Pelomyxa), sometimes found in
+ duck-ponds, or the beautiful Opalina, which always lives in the hind
+ part of the frog's food-canal. If a portion of the living matter of
+ these Protozoa should gather round each of the nuclei, then _that would
+ be the beginning of a body_. It would be still nearer the beginning of a
+ body if division of labour set in, and if there was a setting apart of
+ egg-cells and sperm-cells distinct from body-cells.
+ It was possibly in some such way that animals and plants with a body
+ were first evolved. Two points should be noticed, that body-making is
+ not essentially a matter of size, though it made large size possible.
+ For the body of a many-celled Wheel Animalcule or Rotifer is no bigger
+ than many a Protozoon. Yet the Rotifer--we are thinking of Hydatina--has
+ nine hundred odd cells, whereas the Protozoon has only one, except in
+ forms like Volvox. Secondly, it is a luminous fact that _every
+ many-celled animal from sponge to man that multiplies in the ordinary
+ way begins at the beginning again as a "single cell,"_ the fertilised
+ egg-cell. It is, of course, not an ordinary single cell that develops
+ into an earthworm or a butterfly, an eagle, or a man; it is a cell in
+ which a rich inheritance, the fruition of ages, is somehow condensed;
+ but it is interesting to bear in mind the elementary fact that every
+ many-celled creature, reproduced in the ordinary way and not by budding
+ or the like, starts as a fertilised egg-cell. The coherence of the
+ daughter-cells into which the fertilised egg-cell divides is a
+ reminiscence, as it were, of the primeval coherence of daughter-units
+ that made the first body possible.
+ The Beginning of Sexual Reproduction
+ A freshwater Hydra, growing on the duckweed usually multiplies by
+ budding. It forms daughter-buds, living images of itself; a check comes
+ to nutrition and these daughter-buds go free. A big sea-anemone may
+ divide in two or more parts, which become separate animals. This is
+ asexual reproduction, which means that the multiplication takes place by
+ dividing into two or many portions, and not by liberating egg-cells and
+ sperm-cells. Among animals as among plants, asexual reproduction is very
+ common. But it has great disadvantages, for it is apt to be
+ physiologically expensive, and it is beset with difficulties when the
+ body shows great division of labour, and is very intimately bound into
+ unity. Thus, no one can think of a bee or a bird multiplying by division
+ or by budding. Moreover, if the body of the parent has suffered from
+ injury or deterioration, the result of this is bound to be handed on to
+ the next generation if asexual reproduction is the only method.
+ [Illustration: _Photos: J. J. Ward, F.E.S._
+ A PLANT-LIKE ANIMAL, OR ZOOPHYTE, CALLED OBELIA
+ Consisting of a colony of small polyps, whose stinging tentacles are
+ well shown greatly enlarged in the lower photograph.]
+ [Illustration: _Reproduced by permission of "The Quart. Journ. Mic.
+ Sci."_
+ TRYPANOSOMA GAMBIENSE
+(Very highly magnified.)
+ The microscopic animal Trypanosome, which causes Sleeping Sickness. The
+ study of these organisms has of late years acquired an immense
+ importance on account of the widespread and dangerous maladies to which
+ some of them give rise. It lives in the blood of man, who is infected by
+ the bite of a Tse-tse fly which carries the parasite from some other
+ host.]
+ [Illustration: VOLVOX
+ The Volvox is found in some canals and the like. It is one of the first
+ animals to suggest the beginning of a body. It is a colony of a thousand
+ or even ten thousand cells, but they are all cells of one kind. In
+ _multicellular_ animals the cells are of _different_ kinds with
+ different functions. Each of the ordinary cells (marked 5) has two
+ lashes or flagella. Daughter colonies inside the Parent colony are being
+ formed at 3, 4, and 2. The development of germ-cells is shown at 1.]
+ [Illustration: PROTEROSPONGIA
+ One of the simplest multicellular animals, illustrating the beginning of
+ a body. There is a setting apart of egg-cells and sperm-cells, distinct
+ from body-cells; the collared lashed cells on the margin are different
+ in kind from those farther in. Thus, as in indubitable multicellular
+ animals, division of labour has begun.]
+ Splitting into two or many parts was the old-fashioned way of
+ multiplying, but one of the great steps in evolution was the discovery
+ of a better method, namely, sexual reproduction. The gist of this is
+ simply that during the process of body-building (by the development of
+ the fertilised egg-cell) certain units, _the germ-cells_, do not share
+ in forming ordinary tissues or organs, but remain apart, continuing the
+ full inheritance which was condensed in the fertilised egg-cell. _These
+ cells kept by themselves are the originators of the future reproductive
+ cells of the mature animal_; they give rise to the egg-cells and the
+ sperm-cells.
+ The advantages of this method are great. (1) The new generation is
+ started less expensively, for it is easier to shed germ-cells into the
+ cradle of the water than to separate off half of the body. (2) It is
+ possible to start a great many new lives at once, and this may be of
+ vital importance when the struggle for existence is very keen, and when
+ parental care is impossible. (3) The germ-cells are little likely to be
+ prejudicially affected by disadvantageous dints impressed on the body of
+ the parent--little likely unless the dints have peculiarly penetrating
+ consequences, as in the case of poisons. (4) A further advantage is
+ implied in the formation of two kinds of germ-cells--the ovum or
+ egg-cell, with a considerable amount of building material and often with
+ a legacy of nutritive yolk; the spermatozoon or sperm-cell, adapted to
+ move in fluids and to find the ovum from a distance, thus securing
+ change-provoking cross-fertilisation.
+ The Evolution of Sex
+ Another of the great steps in organic evolution was the differentiation
+ of two different physiological types, the male or sperm-producer and the
+ female or egg-producer. It seems to be a deep-seated difference in
+ constitution, which leads one egg to develop into a male, and another,
+ lying beside it in the nest, into a female. In the case of pigeons it
+ seems almost certain, from the work of Professor Oscar Riddle, that
+ there are two kinds of egg, a male-producing egg and a female-producing
+ egg, which differ in their yolk-forming and other physiological
+ characters.
+ In sea-urchins we often find two creatures superficially
+ indistinguishable, but the one is a female with large ovaries and the
+ other is a male with equally large testes. Here the physiological
+ difference does not affect the body as a whole, but the reproductive
+ organs or gonads only, though more intimate physiology would doubtless
+ discover differences in the blood or in the chemical routine
+ (metabolism). In a large number of cases, however, there are marked
+ superficial differences between the sexes, and everyone is familiar with
+ such contrasts as peacock and peahen, stag and hind. In such cases the
+ physiological difference between the sperm-producer and the
+ ovum-producer, for this is the essential difference, saturates through
+ the body and expresses itself in masculine and feminine structures and
+ modes of behaviour. The expression of the masculine and feminine
+ characters is in some cases under the control of hormones or chemical
+ messengers which are carried by the blood from the reproductive organs
+ throughout the body, and pull the trigger which brings about the
+ development of an antler or a wattle or a decorative plume or a capacity
+ for vocal and saltatory display. In some cases it is certain that the
+ female carries in a latent state the masculine features, but these are
+ kept from expressing themselves by other chemical messengers from the
+ ovary. Of these chemical messengers more must be said later on.
+ Recent research has shown that while the difference between male and
+ female is very deep-rooted, corresponding to a difference in gearing, it
+ is not always clear-cut. Thus a hen-pigeon may be very masculine, and a
+ cock-pigeon very feminine. The difference is in degree, not in kind.
+ What is the meaning of the universal or almost universal inevitableness
+ of death? A Sequoia or "Big Tree" of California has been known to live
+ for over two thousand years, but eventually it died. A centenarian
+ tortoise has been known, and a sea-anemone sixty years of age; but
+ eventually they die. What is the meaning of this apparently inevitable
+ stoppage of bodily life?
+ The Beginning of Natural Death
+ There are three chief kinds of death, (_a_) The great majority of
+ animals come to a violent end, being devoured by others or killed by
+ sudden and extreme changes in their surroundings. (_b_) When an animal
+ enters a new habitat, or comes into new associations with other
+ organisms, it may be invaded by a microbe or by some larger parasite to
+ which it is unaccustomed and to which it can offer no resistance. With
+ many parasites a "live-and-let-live" compromise is arrived at, but new
+ parasites are apt to be fatal, as man knows to his cost when he is
+ bitten by a tse-tse fly which infects him with the microscopic animal (a
+ Trypanosome) that causes Sleeping Sickness. In many animals the
+ parasites are not troublesome as long as the host is vigorous, but if
+ the host is out of condition the parasites may get the upper hand, as in
+ the so-called "grouse disease," and become fatal. (_c_) But besides
+ violent death and microbic (or parasitic) death, there is natural death.
+ This is in great part to be regarded as the price paid for a body. A
+ body worth having implies complexity or division of labour, and this
+ implies certain internal furnishings of a more or less stable kind in
+ which the effects of wear and tear are apt to accumulate. It is not the
+ living matter itself that grows old so much as the framework in which it
+ works--the furnishings of the vital laboratory. There are various
+ processes of rejuvenescence, e.g. rest, repair, change, reorganisation,
+ which work against the inevitable processes of senescence, but sooner or
+ later the victory is with ageing. Another deep reason for natural death
+ is to be found in the physiological expensiveness of reproduction, for
+ many animals, from worms to eels, illustrate natural death as the
+ nemesis of starting new lives. Now it is a very striking fact that to a
+ large degree the simplest animals or Protozoa are exempt from natural
+ death. They are so relatively simple that they can continually
+ recuperate by rest and repair; they do not accumulate any bad debts.
+ Moreover, their modes of multiplying, by dividing into two or many
+ units, are very inexpensive physiologically. It seems that in some
+ measure this bodily immortality of the Protozoa is shared by some simple
+ many-celled animals like the freshwater Hydra and Planarian worms. Here
+ is an interesting chapter in evolution, the evolution of means of
+ evading or staving off natural death. Thus there is the well-known case
+ of the Paloloworm of the coral-reefs where the body breaks up in
+ liberating the germ-cells, but the head-end remains fixed in a crevice
+ of the coral, and buds out a new body at leisure.
+ Along with the evolution of the ways of avoiding death should be
+ considered also the gradual establishment of the length of life best
+ suited to the welfare of the species, and the punctuation of the
+ life-history to suit various conditions.
+ [Illustration: _Photo: J. J. Ward, F.E.S._
+ GREEN HYDRA
+ A little freshwater polyp, about half an inch long, with a crown of
+ tentacles round the mouth. It is seen giving off a bud, a clear
+ illustration of asexual reproduction. When a tentacle touches some small
+ organism the latter is paralysed and drawn into the mouth.]
+ [Illustration: _Photo: J. J. Ward, F.E.S._
+ EARTHWORM
+ Earthworms began the profitable habit of moving with one end of the body
+ always in front, and from worms to man the great majority of animals
+ have bilateral symmetry.]
+ [Illustration: DIAGRAM ILLUSTRATING THE BEGINNING OF INDIVIDUAL LIFE
+1. An immature _sperm_-cell, with 4 chromosomes (nuclear bodies)
+ represented as rods.
+ 2. A mature sperm-cell, with 2 chromosomes.
+ 3. An immature _egg_-cell, with 4 chromosomes represented as curved
+ bodies.
+ 4. A mature egg-cell, with 2 chromosomes.
+ 5. The spermatozoon fertilises the ovum, introducing 2 chromosomes.
+ 6. The fertilised ovum, with 4 chromosomes, 2 of paternal origin and 2
+ of maternal origin.
+ 7. The chromosomes lie at the equator, and each is split longitudinally.
+ The centrosome introduced by the spermatozoon has divided into two
+ centrosomes, one at each pole of the nucleus. These play an important
+ part in the division or segmentation of the egg.
+ 8. The fertilised egg has divided into two cells. Each cell has 2
+ paternal and 2 maternal chromosomes.]
+ [Illustration: _Reproduced from the Smithsonian Report, 1917._
+ GLASS MODEL OF A SEA-ANEMONE
+ A long tubular sea-anemone, with a fine crown of tentacles around the
+ mouth. The suggestion of a flower is very obvious. By means of stinging
+ lassoes on the tentacles minute animals on which it feeds are paralysed
+ and captured for food.]
+ [Illustration: THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH
+ TO MAN
+ The Cerebrum, the seat of intelligence, increases in proportion to the
+ other parts. In mammals it becomes more and more convoluted. The brain,
+ which lies in one plane in fishes, becomes gradually curved on itself.
+ In birds it is more curved than the drawing shows.]
+ Great Acquisitions
+ In animals like sea-anemones and jellyfishes the general symmetry of the
+ body is radial; that is to say, there is no right or left, and the body
+ might be halved along many planes. It is a kind of symmetry well suited
+ for sedentary or for drifting life. But worms began the profitable habit
+ of moving with one end of the body always in front, and from worms to
+ man the great majority of animals have bilateral symmetry. They have a
+ right and a left side, and there is only one cut that halves the body.
+ This kind of symmetry is suited for a more strenuous life than radial
+ animals show; it is suited for pursuing food, for avoiding enemies, for
+ chasing mates. And _with the establishment of bilateral symmetry must be
+ associated the establishment of head-brains_, the beginning of which is
+ to be found in some simple worm-types.
+ Among the other great acquisitions gradually evolved we may notice: a
+ well-developed head with sense-organs, the establishment of large
+ internal surfaces such as the digestive and absorptive wall of the
+ food-canal, the origin of quickly contracting striped muscle and of
+ muscular appendages, the formation of blood as a distributing medium
+ throughout the body, from which all the parts take what they need and to
+ which they also contribute.
+Another very important acquisition, almost confined (so far as is known)
+ to backboned animals, was the evolution of what are called glands of
+ internal secretion, such as the thyroid and the supra-renal. These
+ manufacture subtle chemical substances which are distributed by the
+ blood throughout the body, and have a manifold influence in regulating
+ and harmonising the vital processes. Some of these chemical messengers
+ are called hormones, which stimulate organs and tissues to greater
+ activity; others are called chalones, which put on a brake. Some
+ regulate growth and others rapidly alter the pressure and composition
+ of the blood. Some of them call into active development certain parts of
+ the body which have been, as it were, waiting for an appropriate
+ trigger-pulling. Thus, at the proper time, the milk-glands of a
+ mammalian mother are awakened from their dormancy. This very interesting
+ outcome of evolution will be dealt with in another portion of this work.
+ THE INCLINED PLANE OF ANIMAL BEHAVIOUR
+ Before passing to a connected story of the gradual emergence of higher
+ and higher forms of life in the course of the successive ages--the
+ procession of life, as it may be called--it will be useful to consider
+ the evolution of animal behaviour.
+ Evolution of Mind
+ A human being begins as a microscopic fertilised egg-cell, within which
+ there is condensed the long result of time--Man's inheritance. The long
+ period of nine months before birth, with its intimate partnership
+ between mother and offspring, is passed as it were in sleep, and no one
+ can make any statement in regard to the mind of the unborn child. Even
+ after birth the dawn of mind is as slow as it is wonderful. To begin
+ with, there is in the ovum and early embryo no nervous system at all,
+ and it develops very gradually from simple beginnings. Yet as mentality
+ cannot come in from outside, we seem bound to conclude that the
+ potentiality of it--whatever that means--resides in the individual from
+ the very first. The particular kind of activity known to us as thinking,
+ feeling, and willing is the most intimate part of our experience, known
+ to us directly apart from our senses, and the possibility of that must
+ be implicit in the germ-cell just as the genius of Newton was implicit
+ in a very miserable specimen of an infant. Now what is true of the
+ individual is true also of the race--there is a gradual evolution of
+ that aspect of the living creature's activity which we call mind. We
+ cannot put our finger on any point and say: Before this stage there was
+ no mind. Indeed, many facts suggest the conclusion that wherever there
+ is life there is some degree of mind--even in the plants. Or it might be
+ more accurate to put the conclusion in another way, that the activity we
+ call life has always in some degree an inner or mental aspect.
+ [Illustration: OKAPI AND GIRAFFE
+ The Okapi is one of the great zoölogical discoveries. It gives a good
+ idea of what the Giraffe's ancestors were like. The Okapi was unknown
+ until discovered in 1900 by Sir Harry Johnston in Central Africa, where
+ these strange animals have probably lived in dense forests from time
+ immemorial.]
+ In another part of this book there is an account of the dawn of mind in
+ backboned animals; what we aim at here is an outline of what may be
+ called the inclined plane of animal behaviour.
+ A very simple animal accumulates a little store of potential energy, and
+ it proceeds to expend this, like an explosive, by acting on its
+ environment. It does so in a very characteristic self-preservative
+ fashion, so that it burns without being consumed and explodes without
+ being blown to bits. It is characteristic of the organism that it
+ remains a going concern for a longer or shorter period--its length of
+ life. Living creatures that expended their energy ineffectively or
+ self-destructively would be eliminated in the struggle for existence.
+ When a simple one-celled organism explores a corner of the field seen
+ under a microscope, behaving to all appearance very like a dog scouring
+ a field seen through a telescope, it seems permissible to think of
+ something corresponding to mental endeavour associated with its
+ activity. This impression is strengthened when an amoeba pursues
+ another amoeba, overtakes it, engulfs it, loses it, pursues it again,
+ recaptures it, and so on. What is quite certain is that the behaviour of
+ the animalcule is not like that of a potassium pill fizzing about in a
+ basin of water, nor like the lurching movements of a gun that has got
+ loose and "taken charge" on board ship. Another feature is that the
+ locomotor activity of an animalcule often shows a distinct
+ individuality: it may swim, for instance, in a loose spiral.
+ But there is another side to vital activity besides acting upon the
+ surrounding world; the living creature is acted on by influences from
+ without. The organism acts on its environment; that is the one side of
+ the shield: the environment acts upon the organism; that is the other
+ side. If we are to see life whole we must recognise these two sides of
+ what we call living, and it is missing an important part of the history
+ of animal life if we fail to see that evolution implies becoming more
+ advantageously sensitive to the environment, making more of its
+ influences, shutting out profitless stimuli, and opening more gateways
+ to knowledge. The bird's world is a larger and finer world than an
+ earthworm's; the world means more to the bird than to the worm.
+ The Trial and Error Method
+ Simple creatures act with a certain degree of spontaneity on their
+ environment, and they likewise react effectively to surrounding stimuli.
+ Animals come to have definite "answers back," sometimes several,
+ sometimes only one, as in the case of the Slipper Animalcule, which
+ reverses its cilia when it comes within the sphere of some disturbing
+ influence, retreats, and, turning upon itself tentatively, sets off
+ again in the same general direction as before, but at an angle to the
+ previous line. If it misses the disturbing influence, well and good; if
+ it strikes it again, the tactics are repeated until a satisfactory way
+ out is discovered or the stimulation proves fatal.
+ It may be said that the Slipper Animalcule has but one answer to every
+ question, but there are many Protozoa which have several enregistered
+ reactions. When there are alternative reactions which are tried one
+ after another, the animal is pursuing what is called the trial-and-error
+ method, and a higher note is struck.
+ There is an endeavour after satisfaction, and a trial of answers. When
+ the creature profits by experience to the extent of giving the right
+ answer first, there is the beginning of learning.
+ [Illustration: DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
+ LIKE AN EARTHWORM
+ 1. A sensory nerve-cell (S.C.) on the surface receives a stimulus.
+2. The stimulus travels along the sensatory nerve-fibre (S.F.)
+ 3. The sensory nerve-fibre branches in the nerve-cord.
+ 4. Its branches come into close contact (SY^{1}) with those of an
+ associative or communicating nerve-cell (A.C.).
+ 5. Other branches of the associative cell come into close contact
+ (SY^{2}) with the branches or dendrites of a motor nerve-cell (M.C.).
+ 6. An impulse or command travels along the motor nerve-fibre or axis
+ cylinder of the motor nerve-cell.
+ 7. The motor nerve-fibre ends on a muscle-fibre (M.F.) near the surface.
+ This moves and the reflex action is complete.]
+ [Illustration: _Photo: British Museum_ (_Natural History_).
+ THE YUCCA MOTH
+ The Yucca Moth, emerging from her cocoon, flies at night to a Yucca
+ flower and collects pollen from the stamens, holding a little ball of it
+ in her mouth-parts. She then visits another flower and lays an egg in
+ the seed-box. After this she applies the pollen to the tip of the
+ pistil, thus securing the fertilisation of the flower and the growth of
+ the ovules in the pod. Yucca flowers in Britain do not produce seeds
+ because there are no Yucca Moths.]
+ [Illustration: INCLINED PLANE OF ANIMAL BEHAVIOUR
+ Diagram illustrating animal behaviour. The main line represents the
+ general life of the creature. On the upper side are activities implying
+ initiative; on the lower side actions which are almost automatic.
+ _Upper Side._--I. Energetic actions. II. Simple tentatives. III.
+ Trial-and-error methods. IV. Non-intelligent experiments. V.
+ Experiential "learning." VI. Associative "learning." VII. Intelligent
+ behaviour. VIII. Rational conduct (man).
+ _Lower Side._--1. Reactions to environment. 2. Enregistered reactions.
+ 3. Simple reflex actions. 4. Compound reflex actions. 5. Tropisms. 6.
+ Enregistered rhythms. 7. Simple instincts. 8. Chain instincts. 9.
+ Instinctive activities influenced by intelligence. 10. Subconscious
+ cerebration at a high level (man).]
+ [Illustration: _Photo: J. J. Ward, F.E.S._
+ VENUS' FLY-TRAP
+ One of the most remarkable plants in the world, which captures its prey
+ by means of a trap formed from part of its leaf. It has been induced to
+ snap at and hold a bristle. If an insect lighting on the leaf touches
+ one of six very sensitive hairs, which pull the trigger of the movement,
+ the two halves of the leaf close rapidly and the fringing teeth on the
+ margin interlock, preventing the insect's escape. Then follows an
+ exudation of digestive juice.]
+ [Illustration: _Reproduced by permission from "The Wonders of Instinct"
+ by J. H. Fabre._
+ A SPIDER SUNNING HER EGGS
+ A kind of spider, called Lycosa, lying head downwards at the edge of her
+ nest, and holding her silken cocoon--the bag containing the eggs--up
+ towards the sun in her hindmost pair of legs. This extraordinary
+ proceeding is believed to assist in the hatching.]
+ Reflex Actions
+ Among simple multicellular animals, such as sea-anemones, we find the
+ beginnings of reflex actions, and a considerable part of the behaviour
+ of the lower animals is reflex. That is to say, there are laid down in
+ the animal in the course of its development certain pre-arrangements of
+ nerve-cells and muscle-cells which secure that a fit and proper answer
+ is given to a frequently recurrent stimulus. An earthworm half out of
+ its burrow becomes aware of the light tread of a thrush's foot, and
+ jerks itself back into its hole before anyone can say "reflex action."
+ What is it that happens?
+ Certain sensory nerve-cells in the earthworm's skin are stimulated by
+ vibrations in the earth; the message travels down a sensory nerve-fibre
+ from each of the stimulated cells and enters the nerve-cord. The sensory
+ fibres come into vital connection with branches of intermediary,
+ associative, or communicating cells, which are likewise connected with
+ motor nerve-cells. To these the message is thus shunted. From the motor
+ nerve-cells an impulse or command travels by motor nerve-fibres, one
+ from each cell, to the muscles, which contract. If this took as long to
+ happen as it takes to describe, even in outline, it would not be of much
+ use to the earthworm. But the motor answer follows the sensory stimulus
+ almost instantaneously. The great advantage of establishing or
+ enregistering these reflex chains is that the answers are practically
+ ready-made or inborn, not requiring to be learned. It is not necessary
+ that the brain should be stimulated if there is a brain; nor does the
+ animal will to act, though in certain cases it may by means of higher
+ controlling nerve-centres keep the natural reflex response from being
+ given, as happens, for instance, when we control a cough or a sneeze on
+ some solemn occasion. The evolutionary method, if we may use the
+ expression, has been to enregister ready-made responses; and as we
+ ascend the animal kingdom, we find reflex actions becoming complicated
+ and often linked together, so that the occurrence of one pulls the
+ trigger of another, and so on in a chain. The behaviour of the
+ insectivorous plant called Venus's fly-trap when it shuts on an insect
+ is like a reflex action in an animal, but plants have no definite
+ nervous system.
+ What are Called Tropisms
+ A somewhat higher level on the inclined plane is illustrated by what are
+ called "tropisms," obligatory movements which the animal makes,
+ adjusting its whole body so that physiological equilibrium results in
+ relation to gravity, pressure, currents, moisture, heat, light,
+ electricity, and surfaces of contact. A moth is flying past a candle;
+the eye next the light is more illumined than the other; a physiological
+inequilibrium results, affecting nerve-cells and muscle-cells; the
+outcome is that the moth automatically adjusts its flight so that both
+eyes become equally illumined; in doing this it often flies into the
+candle.
+It may seem bad business that the moth should fly into the candle, but
+the flame is an utterly artificial item in its environment to which no
+one can expect it to be adapted. These tropisms play an important rôle
+in animal behaviour.
+Instinctive Behaviour
+On a higher level is instinctive behaviour, which reaches such
+remarkable perfection in ants, bees, and wasps. In its typical
+expression instinctive behaviour depends on inborn capacities; it does
+not require to be learned; it is independent of practice or experience,
+ though it may be improved by both; it is shared equally by all members
+ of the species of the same sex (for the female's instincts are often
+ different from the male's); it refers to particular conditions of life
+ that are of vital importance, though they may occur only once in a
+ lifetime. The female Yucca Moth emerges from the cocoon when the Yucca
+ flower puts forth its bell-like blossoms. She flies to a flower,
+ collects some pollen from the stamens, kneads it into a pill-like ball,
+ and stows this away under her chin. She flies to an older Yucca flower
+ and lays her eggs in some of the ovules within the seed-box, but before
+ she does so she has to deposit on the stigma the ball of pollen. From
+ this the pollen-tubes grow down and the pollen-nucleus of a tube
+ fertilises the egg-cell in an ovule, so that the possible seeds become
+ real seeds, for it is only a fraction of them that the Yucca Moth has
+ destroyed by using them as cradles for her eggs. Now it is plain that
+ the Yucca Moth has no individual experience of Yucca flowers, yet she
+ secures the continuance of her race by a concatenation of actions which
+ form part of her instinctive repertory.
+ From a physiological point of view instinctive behaviour is like a chain
+ of compound reflex actions, but in some cases, at least, there is reason
+ to believe that the behaviour is suffused with awareness and backed by
+ endeavour. This is suggested in exceptional cases where the stereotyped
+ routine is departed from to meet exceptional conditions. It should also
+ be noted that just as ants, hive bees, and wasps exhibit in most cases
+ purely instinctive behaviour, but move on occasion on the main line of
+ trial and error or of experimental initiative, so among birds and
+ mammals the intelligent behaviour is sometimes replaced by instinctive
+ routine. Perhaps there is no instinctive behaviour without a spice of
+ intelligence, and no intelligent behaviour without an instinctive
+ element. The old view that instinctive behaviour was originally
+ intelligent, and that instinct is "lapsed intelligence," is a tempting
+ one, and is suggested by the way in which habitual intelligent actions
+ cease in the individual to require intelligent control, but it rests on
+ the unproved hypothesis that the acquisitions of the individual can be
+ entailed on the race. It is almost certain that instinct is on a line of
+ evolution quite different from intelligence, and that it is nearer to
+ the inborn inspirations of the calculating boy or the musical genius
+ than to the plodding methods of intelligent learning.
+ Animal Intelligence
+ The higher reaches of the inclined plane of behaviour show intelligence
+ in the strict sense. They include those kinds of behaviour which cannot
+ be described without the suggestion that the animal makes some sort of
+ perceptual inference, not only profiting by experience but learning by
+ ideas. Such intelligent actions show great individual variability; they
+ are plastic and adjustable in a manner rarely hinted at in connection
+ with instincts where routine cannot be departed from without the
+ creature being nonplussed; they are not bound up with particular
+ circumstances as instinctive actions are, but imply an appreciative
+ awareness of relations.
+ When there is an experimenting with general ideas, when there is
+ _conceptual_ as contrasted with _perceptual_ inference, we speak of
+ Reason, but there is no evidence of this below the level of man. It is
+ not, indeed, always that we can credit man with rational conduct, but he
+ has the possibility of it ever within his reach.
+ Animal instinct and intelligence will be illustrated in another part of
+ this work. We are here concerned simply with the general question of the
+ evolution of behaviour. There is a main line of tentative experimental
+ behaviour both below and above the level of intelligence, and it has
+ been part of the tactics of evolution to bring about the hereditary
+ enregistration of capacities of effective response, the advantages being
+ that the answers come more rapidly and that the creature is left free,
+ if it chooses, for higher adventures.
+ There is no doubt as to the big fact that in the course of evolution
+ animals have shown an increasing complexity and masterfulness of
+ behaviour, that they have become at once more controlled and more
+ definitely free agents, and that the inner aspect of the
+ behaviour--experimenting, learning, thinking, feeling, and willing--has
+ come to count for more and more.
+ Evolution of Parental Care
+ Mammals furnish a crowning instance of a trend of evolution which
+ expresses itself at many levels--the tendency to bring forth the young
+ at a well-advanced stage and to an increase of parental care associated
+ with a decrease in the number of offspring. There is a British starfish
+ called _Luidia_ which has two hundred millions of eggs in a year, and
+ there are said to be several millions of eggs in conger-eels and some
+ other fishes. These illustrate the spawning method of solving the
+ problem of survival. Some animals are naturally prolific, and the number
+ of eggs which they sow broadcast in the waters allows for enormous
+ infantile mortality and obviates any necessity for parental care.
+ But some other creatures, by nature less prolific, have found an
+ entirely different solution of the problem. They practise parental care
+ and they secure survival with greatly economised reproduction. This is a
+ trend of evolution particularly characteristic of the higher animals. So
+ much so that Herbert Spencer formulated the generalisation that the size
+ and frequency of the animal family is inverse ratio to the degree of
+ evolution to which the animal has attained.
+ Now there are many different methods of parental care which secure the
+ safety of the young, and one of these is called viviparity. The young
+ ones are not liberated from the parent until they are relatively well
+ advanced and more or less able to look after themselves. This gives the
+ young a good send-off in life, and their chances of death are greatly
+ reduced. In other words, the animals that have varied in the direction
+ of economised reproduction may keep their foothold in the struggle for
+ existence if they have varied at the same time in the direction of
+ parental care. In other cases it may have worked the other way round.
+ In the interesting archaic animal called _Peripatus_, which has to face
+ a modern world too severe for it, one of the methods of meeting the
+ environing difficulties is the retention of the offspring for many
+ months within the mother, so that it is born a fully-formed creature.
+ There are only a few offspring at a time, and, although there are
+ exceptional cases like the summer green-flies, which are very prolific
+ though viviparous, the general rule is that viviparity is associated
+ with a very small family. The case of flowering plants stands by itself,
+ for although they illustrate a kind of viviparity, the seed being
+ embryos, an individual plant may have a large number of flowers and
+ therefore a huge family.
+ Viviparity naturally finds its best illustrations among terrestrial
+ animals, where the risks to the young life are many, and it finds its
+ climax among mammals.
+ Now it is an interesting fact that the three lowest mammals, the
+ Duckmole and two Spiny Ant-eaters, lay eggs, i.e. are oviparous; that
+ the Marsupials, on the next grade, bring forth their young, as it were,
+ prematurely, and in most cases stow them away in an external pouch;
+while all the others--the Placentals--show a more prolonged ante-natal
+life and an intimate partnership between the mother and the unborn
+young.
+There is another way of looking at the sublime process of evolution. It
+has implied a mastery of all the possible haunts of life; it has been a
+progressive conquest of the environment.
+1. It is highly probable that living organisms found their foothold in
+the stimulating conditions of the shore of the sea--the shallow water,
+ brightly illumined, seaweed-growing shelf fringing the Continents. This
+ littoral zone was a propitious environment where sea and fresh water,
+ earth and air all meet, where there is stimulating change, abundant
+ oxygenation and a copious supply of nutritive material in what the
+ streams bring down and in the rich seaweed vegetation.
+ [Illustration: THE HOATZIN INHABITS BRITISH GUIANA
+ The newly hatched bird has claws on its thumb and first finger and so is
+ enabled to climb on the branches of trees with great dexterity until
+ such time as the wings are strong enough to sustain it in flight.]
+ [Illustration: _Photograph, from the British Museum (Natural History),
+ of a drawing by Mr. E. Wilson._
+ PERIPATUS
+ A widely distributed old-fashioned type of animal, somewhat like a
+ permanent caterpillar. It has affinities both with worms and with
+ insects. It has a velvety skin, minute diamond-like eyes, and short
+ stump-like legs. A defenceless, weaponless animal, it comes out at
+ night, and is said to capture small insects by squirting jets of slime
+ from its mouth.]
+ [Illustration: _Photo: W. S. Berridge, F.Z.S._
+ ROCK KANGAROO CARRYING ITS YOUNG IN A POUCH
+ The young are born so helpless that they cannot even suck. The mother
+ places them in the external pouch, and fitting their mouths on the teats
+ injects the milk. After a time the young ones go out and in as they
+ please.]
+ It is not an easy haunt of life, but none the worse for that, and it is
+ tenanted to-day by representatives of practically every class of animals
+ from infusorians to seashore birds and mammals.
+ The Cradle of the Open Sea
+ 2. The open-sea or pelagic haunt includes all the brightly illumined
+ surface waters beyond the shallow water of the shore area.
+ It is perhaps the easiest of all the haunts of life, for there is no
+ crowding, there is considerable uniformity, and an abundance of food for
+ animals is afforded by the inexhaustible floating "sea-meadows" of
+ microscopic Algæ. These are reincarnated in minute animals like the
+ open-sea crustaceans, which again are utilised by fishes, these in turn
+ making life possible for higher forms like carnivorous turtles and
+ toothed whales. It is quite possible that the open sea was the original
+ cradle of life and perhaps Professor Church is right in picturing a long
+ period of pelagic life before there was any sufficiently shallow water
+ to allow the floating plants to anchor. It is rather in favour of this
+ view that many shore animals such as crabs and starfishes, spend their
+ youthful stages in the relatively safe cradle of the open sea, and only
+ return to the more strenuous conditions of their birthplace after they
+ have gained considerable strength of body. It is probably safe to say
+ that the honour of being the original cradle of life lies between the
+ shore of the sea and the open sea.
+ The Great Deeps
+ 3. A third haunt of life is the floor of the Deep Sea, the abyssal area,
+ which occupies more than a half of the surface of the globe. It is a
+ region of extreme cold--an eternal winter; of utter darkness--an eternal
+ night--relieved only by the fitful gleams of "phosphorescent" animals;
+ of enormous pressure--2-1/2 tons on the square inch at a depth of 2,500
+ fathoms; of profound calm, unbroken silence, immense monotony. And as
+ there are no plants in the great abysses, the animals must live on one
+ another, and, in the long run, on the rain of moribund animalcules which
+ sink from the surface through the miles of water. It seems a very
+ unpromising haunt of life, but it is abundantly tenanted, and it gives
+ us a glimpse of the insurgent nature of the living creature that the
+ difficulties of the Deep Sea should have been so effectively conquered.
+ It is probable that the colonising of the great abysses took place in
+ relatively recent times, for the fauna does not include many very
+ antique types. It is practically certain that the colonisation was due
+ to littoral animals which followed the food-débris, millennium after
+ millennium, further and further down the long slope from the shore.
+ The Freshwaters
+ 4. A fourth haunt of life is that of the freshwaters, including river
+ and lake, pond and pool, swamp and marsh. It may have been colonised by
+ gradual migration up estuaries and rivers, or by more direct passage
+ from the seashore into the brackish swamp. Or it may have been in some
+ cases that partially landlocked corners of ancient seas became gradually
+ turned into freshwater basins. The animal population of the freshwaters
+ is very representative, and is diversely adapted to meet the
+ characteristic contingencies--the risk of being dried up, the risk of
+ being frozen hard in winter, and the risk of being left high and dry
+ after floods or of being swept down to the sea.
+ Conquest of the Dry Land
+ 5. The terrestrial haunt has been invaded age after age by contingents
+ from the sea or from the freshwaters. We must recognise the worm
+ invasion, which led eventually to the making of the fertile soil, the
+ invasion due to air-breathing Arthropods, which led eventually to the
+ important linkage between flowers and their insect visitors, and the
+ invasion due to air-breathing Amphibians, which led eventually to the
+ higher terrestrial animals and to the development of intelligence and
+ family affection. Besides these three great invasions, there were minor
+ ones such as that leading to land-snails, for there has been a
+ widespread and persistent tendency among aquatic animals to try to
+ possess the dry land.
+ Getting on to dry land had a manifold significance.
+ It implied getting into a medium with a much larger supply of oxygen
+ than there is dissolved in the water. But the oxygen of the air is more
+ difficult to capture, especially when the skin becomes hard or well
+ protected, as it is almost bound to become in animals living on dry
+ ground. Thus this leads to the development of _internal surfaces_, such
+ as those of lungs, where the oxygen taken into the body may be absorbed
+ by the blood. In most animals the blood goes to the surface of
+ oxygen-capture; but in insects and their relatives there is a different
+ idea--of taking the air to the blood or in greater part to the area of
+ oxygen-combustion, the living tissues. A system of branching air-tubes
+ takes air into every hole and corner of the insect's body, and this
+ thorough aeration is doubtless in part the secret of the insect's
+ intense activity. The blood never becomes impure.
+ The conquest of the dry land also implied a predominance of that kind of
+ locomotion which may be compared to punting, when the body is pushed
+ along by pressing a lever against a hard substratum. And it also
+ followed that with few exceptions the body of the terrestrial animal
+ tended to be compact, readily lifted off the ground by the limbs or
+ adjusted in some other way so that there may not be too large a surface
+ trailing on the ground. An animal like a jellyfish, easily supported in
+ the water, would be impossible on land. Such apparent exceptions as
+ earthworms, centipedes, and snakes are not difficult to explain, for the
+ earthworm is a burrower which eats its way through the soil, the
+ centipede's long body is supported by numerous hard legs, and the snake
+ pushes itself along by means of the large ventral scales to which the
+ lower ends of very numerous ribs are attached.
+ Methods of Mastering the Difficulties of Terrestrial Life
+ A great restriction attendant on the invasion of the dry land is that
+ locomotion becomes limited to one plane, namely, the surface of the
+ earth. This is in great contrast to what is true in the water, where the
+ animal can move up or down, to right or to left, at any angle and in
+ three dimensions. It surely follows from this that the movements of land
+ animals must be rapid and precise, unless, indeed, safety is secured in
+ some other way. Hence it is easy to understand why most land animals
+ have very finely developed striped muscles, and why a beetle running on
+ the ground has far more numerous muscles than a lobster swimming in the
+ sea.
+ Land animals were also handicapped by the risks of drought and of frost,
+ but these were met by defences of the most diverse description, from the
+ hairs of woolly caterpillars to the fur of mammals, from the carapace of
+ tortoises to the armour of armadillos. In other cases, it is hardly
+ necessary to say, the difficulties may be met in other ways, as frogs
+ meet the winter by falling into a lethargic state in some secluded
+ retreat.
+ Another consequence of getting on to dry land is that the eggs or young
+ can no longer be set free anyhow, as is possible when the animal is
+ surrounded by water, which is in itself more or less of a cradle. If the
+ eggs were laid or the young liberated on dry ground, the chances are
+ many that they would be dried up or devoured. So there are numerous ways
+ in which land animals secure the safety of their young, e.g. by burying
+ them in the ground, or by hiding them in nests, or by carrying them
+ about for a prolonged period either before or after birth. This may mean
+ great safety for the young, this may make it possible to have only a
+ small family, and this may tend to the evolution of parental care and
+ the kindly emotions. Thus it may be understood that from the conquest of
+ the land many far-reaching consequences have followed.
+ [Illustration: _Photo: Rischgitz._
+PROFESSOR THOMAS HENRY HUXLEY (1825-95)
+ One of the most distinguished of zoologists, with unsurpassed gifts as a
+ teacher and expositor. He did great service in gaining a place for
+ science in ordinary education and in popular estimation. No one
+ championed Evolutionism with more courage and skill.]
+ [Illustration: BARON CUVIER, 1769-1832
+ One of the founders of modern Comparative Anatomy. A man of gigantic
+ intellect, who came to Paris as a youth from the provinces, and became
+ the director of the higher education of France and a peer of the Empire.
+ He was opposed to Evolutionist ideas, but he had anatomical genius.]
+ [Illustration: AN ILLUSTRATION SHOWING VARIOUS METHODS OF FLYING AND
+ SWOOPING
+ Gull, with a feather-wing, a true flier. Fox-bat, with a skin-wing, a
+ true flier. Flying Squirrel, with a parachute of skin, able to swoop
+ from tree to tree, but not to fly. Flying Fish, with pectoral fins used
+ as volplanes in a great leap due to the tail. To some extent able to
+ sail in albatros fashion.]
+ Finally, it is worth dwelling on the risks of terrestrial life, because
+ they enable us better to understand why so many land animals have become
+ burrowers and others climbers of trees, why some have returned to the
+ water and others have taken to the air. It may be asked, perhaps, why
+ the land should have been colonised at all when the risks and
+ difficulties are so great. The answer must be that necessity and
+ curiosity are the mother and father of invention. Animals left the water
+ because the pools dried up, or because they were overcrowded, or because
+ of inveterate enemies, but also because of that curiosity and spirit of
+ adventure which, from first to last, has been one of the spurs of
+ progress.
+ Conquering the Air
+ 6. The last great haunt of life is the air, a mastery of which must be
+ placed to the credit of insects, Pterodactyls, birds, and bats. These
+ have been the successes, but it should be noted that there have been
+ many brilliant failures, which have not attained to much more than
+ parachuting. These include the Flying Fishes, which take leaps from the
+ water and are carried for many yards and to considerable heights,
+ holding their enlarged pectoral fins taut or with little more than a
+ slight fluttering. There is a so-called Flying Frog (_Rhacophorus_) that
+ skims from branch to branch, and the much more effective Flying Dragon
+ (_Draco volans_) of the Far East, which has been mentioned already.
+ Among mammals there are Flying Phalangers, Flying Lemurs, and more
+ besides, all attaining to great skill as parachutists, and illustrating
+ the endeavour to master the air which man has realised in a way of his
+ own.
+ The power of flight brings obvious advantages. A bird feeding on the
+ ground is able to evade the stalking carnivore by suddenly rising into
+ the air; food and water can be followed rapidly and to great distances;
+ the eggs or the young can be placed in safe situations; and birds in
+ their migrations have made a brilliant conquest both of time and space.
+ Many of them know no winter in their year, and the migratory flight of
+ the Pacific Golden Plover from Hawaii to Alaska and back again does not
+ stand alone.
+ THE PROCESSION OF LIFE THROUGH THE AGES
+ The Rock Record
+ How do we know when the various classes of animals and plants were
+ established on the earth? How do we know the order of their appearance
+ and the succession of their advances? The answer is: by reading the Rock
+ Record. In the course of time the crust of the earth has been elevated
+ into continents and depressed into ocean-troughs, and the surface of the
+ land has been buckled up into mountain ranges and folded in gentler
+ hills and valleys. The high places of the land have been weathered by
+ air and water in many forms, and the results of the weathering have been
+ borne away by rivers and seas, to be laid down again elsewhere as
+ deposits which eventually formed sandstones, mudstones, and similar
+ sedimentary rocks. Much of the material of the original crust has thus
+ been broken down and worked up again many times over, and if the total
+ thickness of the sedimentary rocks is added up it amounts, according to
+ some geologists, to a total of 67 miles. In most cases, however, only a
+ small part of this thickness is to be seen in one place, for the
+ deposits were usually formed in limited areas at any one time.
+ The Use of Fossils
+ When the sediments were accumulating age after age, it naturally came
+ about that remains of the plants and animals living at the time were
+ buried, and these formed the fossils by the aid of which it is possible
+ to read the story of the past. By careful piecing together of evidence
+ the geologist is able to determine the order in which the different
+ sedimentary rocks were laid down, and thus to say, for instance, that
+ the Devonian period was the time of the origin of Amphibians. In other
+ cases the geologist utilises the fossils in his attempt to work out the
+ order of the strata when these have been much disarranged. For the
+ simpler fossil forms of any type must be older than those that are more
+ complex. There is no vicious circle here, for the general succession of
+ strata is clear, and it is quite certain that there were fishes before
+ there were amphibians, and amphibians before there were reptiles, and
+ reptiles before there were birds and mammals. In certain cases, e.g. of
+ fossil horses and elephants, the actual historical succession has been
+ clearly worked out.
+ If the successive strata contained good samples of all the plants and
+ animals living at the time when the beds were formed, then it would be
+ easy to read the record of the rocks, but many animals were too soft to
+ become satisfactory fossils, many were eaten or dissolved away, many
+ were destroyed by heat and pressure, so that the rock record is like a
+ library very much damaged by fire and looting and decay.
+ The Geological Time-table
+ The long history of the earth and its inhabitants is conveniently
+ divided into eras. Thus, just as we speak of the ancient, mediæval, and
+ modern history of mankind, so we may speak of Palæozoic, Mesozoic and
+ Cenozoic eras in the history of the earth as a whole.
+ Geologists cannot tell us except in an approximate way how long the
+ process of evolution has taken. One of the methods is to estimate how
+ long has been required for the accumulation of the salts of the sea,
+ for all these have been dissolved out of the rocks since rain began to
+ fall on the earth. Dividing the total amount of saline matter by what is
+ contributed every year in modern times, we get about a hundred million
+ years as the age of the sea. But as the present rate of
+ salt-accumulation is probably much greater than it was during many of
+ the geological periods, the prodigious age just mentioned is in all
+ likelihood far below the mark. Another method is to calculate how long
+ it would take to form the sedimentary rocks, like sandstones and
+ mudstones, which have a _total_ thickness of over fifty miles, though
+ the _local_ thickness is rarely over a mile. As most of the materials
+ have come from the weathering of the earth's crust, and as the annual
+ amount of weathering now going on can be estimated, the time required
+ for the formation of the sedimentary rocks of the world can be
+ approximately calculated. There are some other ways of trying to tell
+ the earth's age and the length of the successive periods, but no
+ certainty has been reached.
+ The eras marked on the table (page 92) as _before the Cambrian_
+ correspond to about thirty-two miles of thickness of strata; and all the
+ subsequent eras with fossil-bearing rocks to a thickness of about
+ twenty-one miles--in itself an astounding fact. Perhaps thirty million
+ years must be allotted to the Pre-Cambrian eras, eighteen to the
+ Palæozoic, nine to the Mesozoic, three to the Cenozoic, making a grand
+ total of sixty millions.
+ The Establishment of Invertebrate Stocks
+ It is an astounding fact that at least half of geological time (the
+ Archæozoic and Proterozoic eras) passed before there were living
+ creatures with parts sufficiently hard to form fossils. In the latter
+ part of the Proterozoic era there are traces of one-celled marine
+ animals (Radiolarians) with shells of flint, and of worms that wallowed
+ in the primal mud. It is plain that as regards the most primitive
+ creatures the rock record tells us little.
+ [Illustration: _From Knipe's "Nebula to Man."_
+ ANIMALS OF THE CAMBRIAN PERIOD e.g. Sponges, Jellyfish, Starfish,
+ Sea-lilies, Water-fleas, and Trilobites]
+ [Illustration: _Photo: J. J. Ward, F.E.S._
+ A TRILOBITE
+ Trilobites were ancient seashore animals, abundant from the Upper
+ Cambrian to the Carboniferous eras. They have no direct descendants
+ to-day. They were jointed-footed animals, allied to Crustaceans and
+ perhaps also to King-crabs. They were able to roll themselves up in
+ their ring-armour.]
+ [Illustration: _Photo: British Museum (Natural History)._
+ THE GAMBIAN MUD-FISH, PROTOPTERUS
+ It can breathe oxygen dissolved in water by its gills; it can also
+ breathe dry air by means of its swim-bladder, which has become a lung.
+ It is a _double-breather_, showing evolution in process. For seven
+ months of the year, the dry season, it can remain inert in the mud,
+ getting air through an open pipe to the surface. When water fills the
+ pools it can use its gills again. Mud-nests or mud encasements with the
+ lung-fish inside have often been brought to Britain and the fish when
+ liberated were quite lively.]
+ [Illustration: THE ARCHÆOPTERYX
+(_After William Leche of Stockholm._)
+ A good restoration of the oldest known bird, Archæopteryx (Jurassic
+ Era). It was about the size of a crow; it had teeth on both jaws; it had
+ claws on the thumb and two fingers; and it had a long lizard-like tail.
+ But it had feathers, proving itself a true bird.]
+ [Illustration: WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS
+ The longest feathers or primaries (PR) are borne by the two fingers (2
+ and 3), and their palm-bones (CMC); the second longest or secondaries
+ are borne by the ulna bone (U) of the fore-arm; there is a separate tuft
+ (AS) on the thumb (TH).]
+ The rarity of direct traces of life in the oldest rocks is partly due to
+ the fact that the primitive animals would be of delicate build, but it
+ must also be remembered that the ancient rocks have been profoundly and
+ repeatedly changed by pressure and heat, so that the traces which did
+ exist would be very liable to obliteration. And if it be asked what
+ right we have to suppose the presence of living creatures in the absence
+ or extreme rarity of fossils, we must point to great accumulations of
+ limestone which indicate the existence of calcareous algæ, and to
+ deposits of iron which probably indicate the activity of iron-forming
+ Bacteria. Ancient beds of graphite similarly suggest that green plants
+ flourished in these ancient days.
+The Era of Ancient Life (Palæozoic)
+ The _Cambrian_ period was the time of the establishment of the chief
+ stocks of backboneless animals such as sponges, jellyfishes, worms,
+ sea-cucumbers, lamp-shells, trilobites, crustaceans, and molluscs. There
+ is something very eloquent in the broad fact that the peopling of the
+ seas had definitely begun some thirty million years ago, for Professor
+ H. F. Osborn points out that in the Cambrian period there was already a
+ colonisation of the shore of the sea, the open sea, and the deep waters.
+ The _Ordovician_ period was marked by abundant representation of the
+ once very successful class of Trilobites--jointed-footed,
+ antenna-bearing, segmented marine animals, with numerous appendages and
+ a covering of chitin. They died away entirely with the end of the
+ Palæozoic era. Also very notable was the abundance of predatory
+ cuttlefishes, the bullies of the ancient seas. But it was in this period
+ that the first backboned animals made their appearance--an epoch-making
+ step in evolution. In other words, true fishes were evolved--destined in
+the course of ages to replace the cuttlefishes (which are mere molluscs)
+ in dominating the seas.
+ In the _Silurian_ period in which the peopling of the seas went on
+ apace, there was the first known attempt at colonising the dry land. For
+ in Silurian rocks there are fossil scorpions, and that implies ability
+ to breathe dry air--by means of internal surfaces, in this case known as
+ lungbooks. It was also towards the end of the Silurian, when a period of
+ great aridity set in, that fishes appeared related to our mud-fishes or
+ double-breathers (Dipnoi), which have lungs as well as gills. This,
+ again, meant utilising dry air, just as the present-day mud-fishes do
+ when the water disappears from the pools in hot weather. The lung-fishes
+ or mud-fishes of to-day are but three in number, one in Queensland, one
+ in South America, and one in Africa, but they are extremely
+ interesting "living fossils," binding the class of fishes to that of
+ amphibians. It is highly probable that the first invasion of the dry
+ land should be put to the credit of some adventurous worms, but the
+ second great invasion was certainly due to air-breathing Arthropods,
+ like the pioneer scorpion we mentioned.
+ [Illustration: PICTORIAL REPRESENTATION OF THE SUCCESSIVE STRATA OF THE
+ EARTH'S CRUST, WITH SUGGESTIONS OF CHARACTERISTIC FOSSILS
+ E.g. Fish and Trilobite in the Devonian (red), a large Amphibian in the
+ Carboniferous (blue), Reptiles in Permian (light red), the first Mammal
+ in the Triassic (blue), the first Bird in the Jurassic (yellow), Giant
+ Reptiles in the Cretaceous (white), then follow the Tertiary strata with
+ progressive mammals, and Quaternary at the top with man and mammoth.]
+ The _Devonian_ period, including that of the Old Red Sandstone, was one
+ of the most significant periods in the earth's history. For it was the
+ time of the establishment of flowering plants upon the earth and of
+ terrestrial backboned animals. One would like to have been the
+ discoverer of the Devonian foot-print of _Thinopus_, the first known
+ Amphibian foot-print--an eloquent vestige of the third great invasion of
+ the dry land. It was probably from a stock of Devonian lung-fishes that
+ the first Amphibians sprang, but it was not till the next period that
+ they came to their own. While they were still feeling their way, there
+ was a remarkable exuberance of shark-like and heavily armoured fishes in
+ the Devonian seas.
+ EVOLUTION OF LAND ANIMALS
+ Giant Amphibians and Coal-measures
+ The _Carboniferous_ period was marked by a mild moist climate and a
+ luxuriant vegetation in the swampy low grounds. It was a much less
+ strenuous time than the Devonian period; it was like a very long summer.
+ There were no trees of the type we see now, but there were forests of
+ club-mosses and horsetails which grew to a gigantic size compared with
+ their pigmy representatives of to-day. In these forests the
+ jointed-footed invaders of the dry land ran riot in the form of
+ centipedes, spiders, scorpions, and insects, and on these the primeval
+ Amphibians fed. The appearance of insects made possible a new linkage of
+ far-reaching importance, namely, the cross-fertilisation of flowering
+ plants by their insect visitors, and from this time onwards it may be
+ said that flowers and their visitors have evolved hand in hand.
+ Cross-fertilisation is much surer by insects than by the wind, and
+ cross-fertilisation is more advantageous than self-fertilisation because
+ it promotes both fertility and plasticity. It was probably in this
+ period that _coloured_ flowers--attractive to insect-visitors--began to
+ justify themselves as beauty became useful, and began to relieve the
+ monotonous green of the horsetail and club-moss forests, which covered
+ great tracts of the earth for millions of years. In the Carboniferous
+ forests there were also land-snails, representing one of the minor
+ invasions of the dry land, tending on the whole to check vegetation.
+ They, too, were probably preyed upon by the Amphibians, some of which
+ attained a large size. Each age has had its giants, and those of the
+ Carboniferous were Amphibians called Labyrinthodonts, some of which were
+ almost as big as donkeys. It need hardly be said that it was in this
+ period that most of the Coal-measures were laid down by the immense
+ accumulation of the spores and debris of the club-moss forests. Ages
+ afterwards, it was given to man to tap this great source of
+ energy--traceable back to the sunshine of millions of years ago. Even
+ then it was true that no plant or animal lives or dies to itself!
+ The Acquisitions of Amphibians.
+ As Amphibians had their Golden Age in the Carboniferous period we may
+ fitly use this opportunity of indicating the advances in evolution which
+ the emergence of Amphibians implied. (1) In the first place the passage
+ from water to dry land was the beginning of a higher and more promiseful
+ life, taxed no doubt by increased difficulties. The natural question
+ rises why animals should have migrated from water to dry land at all
+ when great difficulties were involved in the transition. The answers
+ must be: (_a_) that local drying up of water-basins or elevations of the
+ land surface often made the old haunts untenable; (_b_) that there may
+ have been great congestion and competition in the old quarters; and
+ (_c_) that there has been an undeniable endeavour after well-being
+ throughout the history of animal life. In the same way with mankind,
+ migrations were prompted by the setting in of prolonged drought, by
+ over-population, and by the spirit of adventure. (2) In Amphibians for
+ the first time the non-digitate paired fins of fishes were replaced by
+ limbs with fingers and toes. This implied an advantageous power of
+ grasping, of holding firm, of putting food into the mouth, of feeling
+ things in three dimensions. (3) We cannot be positive in regard to the
+ soft parts of the ancient Amphibians known only as fossils, but if they
+ were in a general way like the frogs and toads, newts and salamanders of
+ the present day, we may say that they made among other acquisitions the
+ following: true ventral lungs, a three-chambered heart, a movable
+ tongue, a drum to the ear, and lids to the eyes. It is very interesting
+ to find that though the tongue of the tadpole has some muscle-fibres in
+ it, they are not strong enough to effect movement, recalling the tongue
+ of fishes, which has not any muscles at all. Gradually, as the tadpole
+ becomes a frog, the muscle-fibres grow in strength, and make it possible
+ for the full-grown creature to shoot out its tongue upon insects. This
+ is probably a recapitulation of what was accomplished in the course of
+ millennia in the history of the Amphibian race. (4) Another acquisition
+ made by Amphibians was a voice, due, as in ourselves, to the rapid
+ passage of air over taut membranes (vocal cords) stretched in the
+ larynx. It is an interesting fact that for millions of years there was
+ upon the earth no sound of life at all, only the noise of wind and wave,
+ thunder and avalanche. Apart from the instrumental music of some
+ insects, perhaps beginning in the Carboniferous, the first vital sounds
+ were due to Amphibians, and theirs certainly was the first voice--surely
+ one of the great steps in organic evolution.
projects / IRC.git / commitdiff