planet. The main mass gave off another ring and another till all theplanets, including the earth, were formed. The central mass persisted asthe sun.

Laplace spoke of his theory, which Kant had anticipated forty-one yearsbefore, with scientific caution: “conjectures which I present with allthe distrust which everything not the result of observation or ofcalculation ought to inspire.” Subsequent research justified hisdistrust, for it has been shown that the original nebula need not havebeen hot and need not have been gaseous. Moreover, there are greatdifficulties in Laplace’s theory of the separation of successive ringsfrom the main mass, and of the condensation of a whirling gaseous ringinto a planet.

So it has come about that the picture of a hot gaseous nebula revolvingas a unit body has given place to other pictures. Thus Sir NormanLockyer pointed out (1890) that the earth is gathering to itselfmillions of meteorites every day; this has been going on for millions ofyears; in distant ages the accretion may have been vastly more rapid andvoluminous; and so the earth has grown! Now the meteoritic contributionsare undoubted, but they require a centre to attract them, and thedifficulty is to account for the beginning of a collecting centre orplanetary nucleus. Moreover, meteorites are sporadic and erratic,scattered hither and thither rather than collecting into unit-bodies. AsProfessor Chamberlin says, “meteorites have rather the characteristicsof the wreckage of some earlier organisation than of the parentage ofour planetary system.” Several other theories have been propounded toaccount for the origin of the earth, but the one that has found mostfavour 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 collectingcentres drawing flying matter into their clutches. Whatever part of theprimitive bolt escaped and scattered was drawn out into independentorbits round the sun, forming the “planetesimals” which behave likeminute planets. These planetesimals formed the food on which the knotssubsequently fed.

The Growth of the Earth

It has been calculated that the newborn earth–the “earth-knot” ofChamberlin’s theory–had a diameter of about 5,500 miles. But it grewby drawing planetesimals into itself until it had a diameter of over8,100 miles at the end of its growing period. Since then it has shrunk,by periodic shrinkages which have meant the buckling up of successiveseries of mountains, and it has now a diameter of 7,918 miles. Butduring the shrinking the earth became more varied.

A sort of slow boiling of the internally hot earth often forced moltenmatter through the cold outer crust, and there came about a gradualassortment of lighter materials nearer the surface and heavier materialsdeeper down. The continents are built of the lighter materials, such asgranites, while the beds of the great oceans are made of the heaviermaterials such as basalts. In limited areas land has often become sea,and sea has often given place to land, but the probability is that thedistinction of the areas corresponding to the great continents andoceans goes back to a very early stage.

The lithosphere is the more or less stable crust of the earth, which mayhave been, to begin with, about fifty miles in thickness. It seems thatthe young earth had no atmosphere, and that ages passed before waterbegan to accumulate on its surface–before, in other words, there wasany hydrosphere. The water came from the earth itself, to begin with,and it was long before there was any rain dissolving out saline matterfrom the exposed rocks and making the sea salt. The weathering of thehigh grounds of the ancient crust by air and water furnished thematerial which formed the sandstones and mudstones and other sedimentaryrocks, which are said to amount to a thickness of over fifty miles inall.

§ 3

Making a Home for Life

It is interesting to inquire how the callous, rough-and-tumbleconditions of the outer world in early days were replaced by others thatallowed of the germination and growth of that tender plant we callLIFE. There are very tough living creatures, but the average organism isill suited for violence. Most living creatures are adapted to mildtemperatures and gentle reactions. Hence the fundamental importance ofthe early atmosphere, heavy with planetesimal dust, in blanketing theearth against intensities of radiance from without, as Chamberlin says,and inequalities of radiance from within. This was the first preparationfor life, but it was an atmosphere without free oxygen. Not lessimportant was the appearance of pools and lakelets, of lakes and seas.Perhaps the early waters covered the earth. And water was the secondpreparation for life–water, that can dissolve a larger variety ofsubstances in greater concentration than any other liquid; water, thatin summer does not readily evaporate altogether from a pond, nor inwinter freeze throughout its whole extent; water, that is such a mobilevehicle and such a subtle cleaver of substances; water, that forms over80 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 thecooling 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 formationof the plastic and permeable materials that build up living creatures.We must not pursue the idea, but it is clear that the stones and mortarof the inanimate world are such that they built a friendly home forlife.

Origin of Living Creatures upon the Earth

During the early chapters of the earth’s history, no living creaturethat we can imagine could possibly have lived there. The temperature wastoo high; there was neither atmosphere nor surface water. Therefore itfollows that at some uncertain, but inconceivably distant date, livingcreatures appeared upon the earth. No one knows how, but it isinteresting to consider possibilities.

[Illustration: _Reproduced from the Smithsonian Report, 1915._

A LIMESTONE CANYON

Many fossils of extinct animals have been found in such rockformations.]

[Illustration: GENEALOGICAL TREE OF ANIMALS

Showing in order of evolution the general relations of the chief classesinto which the world of living things is divided. This scheme representsthe 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 hintof the original ancestors. It looks like a tiny irregular speck ofgreyish jelly, about 1/100th of an inch in diameter. It is commonlyfound gliding on the mud or weeds in ponds, where it engulfs itsmicroscopic food by means of out-flowing lobes (PS). The food vacuole(FV) contains ingested food. From the contractile vacuole (CV) the wastematter is discharged. N is the nucleus, GR, granules.]

From ancient times it has been a favourite answer that the dust of theearth may have become living in a way which is outside scientificdescription. This answer forecloses the question, and it is far too soonto do that. Science must often say “Ignoramus”: Science should be slowto say “Ignorabimus.”

A second position held by Helmholtz, Lord Kelvin, and others, suggeststhat minute living creatures may have come to the earth from elsewhere,in the cracks of a meteorite or among cosmic dust. It must be rememberedthat seeds can survive prolonged exposure to very low temperatures; thatspores of bacteria can survive high temperature; that seeds of plantsand germs of animals in a state of “latent life” can survive prolongeddrought and absence of oxygen. It is possible, according to Berthelot,that as long as there is not molecular disintegration vital activitiesmay be suspended for a time, and may afterwards recommence whenappropriate conditions are restored. Therefore, one should be slow tosay that a long journey through space is impossible. The obviouslimitation of Lord Kelvin’s theory is that it only shifts the problem ofthe origin of organisms (i.e. living creatures) from the earth toelsewhere.

The third answer is that living creatures of a very simple sort may haveemerged on the earth’s surface from not-living material, e.g. from somesemi-fluid carbon compounds activated by ferments. The tenability ofthis 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 cleversynthetic chemist, but there seems to be a tendency to complexity.Corpuscles form atoms, atoms form molecules, small molecules largeones.

Various concrete suggestions have been made in regard to the possibleorigin of living matter, which will be dealt with in a later chapter. Sofar as we know of what goes on to-day, there is no evidence ofspontaneous generation; organisms seem always to arise from pre-existingorganisms of the same kind; where any suggestion of the contrary hasbeen fancied, there have been flaws in the experimenting. But it is onething to accept the verdict “omne vivum e vivo” as a fact to whichexperiment has not yet discovered an exception and another thing tomaintain that this must always have been true or must always remaintrue.

If the synthetic chemists should go on surpassing themselves, ifsubstances like white of egg should be made artificially, and if weshould get more light on possible steps by which simple living creaturesmay have arisen from not-living materials, this would not greatly affectour general outlook on life, though it would increase our appreciationof what is often libelled as “inert” matter. If the dust of the earthdid naturally give rise very long ago to living creatures, if they arein a real sense born of her and of the sunshine, then the whole worldbecomes more continuous and more vital, and all the inorganic groaningand travailing becomes more intelligible.

§ 4

The First Organisms upon the Earth

We cannot have more than a speculative picture of the first livingcreatures upon the earth or, rather, in the waters that covered theearth. A basis for speculation is to be found, however, in the simplestcreatures living to-day, such as some of the bacteria and one-celledanimalcules, especially those called Protists, which have not taken anyvery definite step towards becoming either plants or animals. No one canbe sure, but there is much to be said for the theory that the firstcreatures were microscopic globules of living matter, not unlike thesimplest bacteria of to-day, but able to live on air, water, anddissolved salts. From such a source may have originated a race ofone-celled marine organisms which were able to manufacture chlorophyll,or something like chlorophyll, that is to say, the green pigment whichmakes it possible for plants to utilise the energy of the sunlight inbreaking up carbon dioxide and in building up (photosynthesis) carboncompounds like sugars and starch. These little units were probablyencased in a cell-wall of cellulose, but their boxed-in energy expresseditself in the undulatory movement of a lash or flagellum, by means ofwhich they propelled themselves energetically through the water. Thereare many similar organisms to-day, mostly in water, but some ofthem–simple one-celled plants–paint the tree-stems and even thepaving-stones green in wet weather. According to Prof. A. H. Churchthere was a long chapter in the history of the earth when the sea thatcovered everything teemed with these green flagellates–the originatorsof the Vegetable Kingdom.

On another tack, however, there probably evolved a series of simplepredatory creatures, not able to build up organic matter from air,water, and salts, but devouring their neighbours. These units were notclosed in with cellulose, but remained naked, with their living matteror protoplasm flowing out in changeful processes, such as we see in theAmoebæ in the ditch or in our own white blood corpuscles and otheramoeboid cells. These were the originators of the animal kingdom. Thusfrom very simple Protists the first animals and the first plants mayhave arisen. All were still very minute, and it is worth rememberingthat had there been any scientific spectator after our kind upon theearth during these long ages, he would have lamented the entire absenceof life, although the seas were teeming. The simplest forms of life andthe protoplasm which Huxley called the physical basis of life will bedealt 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 OFSEX–BEGINNING OF NATURAL DEATH

§ 1

The Contrast between Plants and Animals

However it may have come about, there is no doubt at all that one of thefirst great steps in Organic Evolution was the forking of thegenealogical tree into Plants and Animals–the most important parting ofthe ways in the whole history of Nature.

Typical plants have chlorophyll; they are able to feed at a low chemicallevel on air, water, and salts, using the energy of the sunlight intheir photosynthesis. They have their cells boxed in by cellulose walls,so that their opportunities for motility are greatly restricted. Theymanufacture much more nutritive material than they need, and live farbelow their income. They have no ready way of getting rid of anynitrogenous waste matter that they may form, and this probably helps tokeep them sluggish.

Animals, on the other hand, feed at a high chemical level, on thecarbohydrates (e.g. starch and sugar), fats, and proteins (e.g. gluten,albumin, casein) which are manufactured by other animals, or to beginwith, by plants. Their cells have not cellulose walls, nor in most casesmuch wall of any kind, and motility in the majority is unrestricted.Animals live much more nearly up to their income. If we could make foran animal and a plant of equal weight two fractions showing the ratio ofthe 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 correspondingratio for the animal. In other words, animals take the munitions whichplants laboriously manufacture and explode them in locomotion andwork; and the entire system of animate nature depends upon thephotosynthesis 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 SMALLSEA-ANEMONE-LIKE POLYPS, EACH OF WHICH FORMS FROM THE SALTS OF THE SEA ASKELETON OR SHELL OF LIME

The wonderful mass of corals, which are very beautiful, are the skeletonremains of hundreds of these little creatures.]

[Illustration: _Photo: J. J. Ward, F.E.S._

THE INSET CIRCLE SHOWS A GROUP OF CHALK-FORMING ANIMALS, ORFORAMINIFERA, EACH ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD

They form a great part of the chalk cliffs of Dover and similar depositswhich 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 OUTFLOWINGNETWORK OF LIVING MATTER, ALONG WHICH GRANULES ARE CONTINUALLYTRAVELLING, AND BY WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN

_Reproduced by permission of the Natural History Museum_ (_after MaxSchultze_).]

As the result of much more explosive life, animals have to deal withmuch in the way of nitrogenous waste products, the ashes of the livingfire, but these are usually got rid of very effectively, e.g. in thekidney filters, and do not clog the system by being deposited ascrystals and the like, as happens in plants. Sluggish animals likesea-squirts which have no kidneys are exceptions that prove the rule,and it need hardly be said that the statements that have been made inregard to the contrasts between plants and animals are generalstatements. There is often a good deal of the plant about the animal, asin sedentary sponges, zoophytes, corals, and sea-squirts, and there isoften a little of the animal about the plant, as we see in the movementsof all shoots and roots and leaves, and occasionally in the parts of theflower. But the important fact is that on the early forking of thegenealogical tree, i.e. the divergence of plants and animals, theredepended and depends all the higher life of the animal kingdom, not tospeak of mankind. The continuance of civilisation, the upkeep of thehuman and animal population of the globe, and even the supply of oxygento the air we breathe, depend on the silent laboratories of the greenleaves, which are able with the help of the sunlight to use carbonicacid, water, and salts to build up the bread of life.

§ 2

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 inthe universal Open Sea. But contraction of the earth’s crust broughtabout elevations and depressions of the sea-floor, and in places thesolid substratum was brought near enough the surface to allow thefloating plants to begin to settle down without getting out of thelight. This is how Professor Church pictures the beginning of a fixedvegetation–a very momentous step in evolution. It was perhaps amongthis early vegetation that animals had their first successes. As thefloor of the sea in these shallow areas was raised higher and higherthere was a beginning of dry land. The sedentary plants already spokenof were the ancestors of the shore seaweeds, and there is no doubt thatwhen we go down at the lowest tide and wade cautiously out among thejungle of vegetation only exposed on such occasions we are getting aglimpse 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 isthat the very simplest of them may give us some hint of the simplicityof the original first animals. For it is quite certain that the vastmajority of the Protozoa to-day are far too complicated to be thought ofas primitive. Though most of them are microscopic, each is an animalcomplete in itself, with the same fundamental bodily attributes as aremanifested in ourselves. They differ from animals of higher degree innot being built up of the unit areas or corpuscles called cells. Theyhave no cells, no tissues, no organs, in the ordinary acceptation ofthese words, but many of them show a great complexity of internalstructure, far exceeding that of the ordinary cells that build up thetissues of higher animals. They are complete living creatures which havenot gone in for body-making.

In the dim and distant past there was a time when the only animals wereof the nature of Protozoa, and it is safe to say that one of the greatsteps in evolution was the establishment of three great types ofProtozoa: (_a_) Some were very active, the Infusorians, like the slipperanimalcule, the night-light (Noctiluca), which makes the seasphosphorescent at night, and the deadly Trypanosome, which causesSleeping Sickness. (_b_) Others were very sluggish, the parasiticSporozoa, like the malaria organism which the mosquito introduces intoman’s body. (_c_) Others were neither very active nor very passive, theRhizopods, with out-flowing processes of living matter. This amoeboidline of evolution has been very successful; it is represented by theRhizopods, such as Amoebæ and the chalk-forming Foraminifera and theexquisitely beautiful flint-shelled Radiolarians of the open sea. Theyhave their counterparts in the amoeboid cells of most multicellularanimals, such as the phagocytes which migrate about in the body,engulfing and digesting intruding bacteria, serving as sappers andminers when something has to be broken down and built up again, andperforming other useful offices.

§ 3

The Making of a Body

The great naturalist Louis Agassiz once said that the biggest gulf inOrganic Nature was that between the unicellular and the multicellularanimals (Protozoa and Metazoa). But the gulf was bridged very long agowhen sponges, stinging animals, and simple worms were evolved, andshowed, for the first time, a “body.” What would one not give to be ableto account for the making of a body, one of the great steps inevolution! No one knows, but the problem is not altogether obscure.

When an ordinary Protozoon or one-celled animal divides into two ormore, which is its way of multiplying, the daughter-units thus formedfloat apart and live independent lives. But there are a few Protozoa inwhich the daughter-units are not quite separated off from one another,but remain coherent. Thus Volvox, a beautiful green ball, found in somecanals and the like, is a colony of a thousand or even ten thousandcells. It has almost formed a body! But in this “colony-making”Protozoon, and in others like it, the component cells are all of onekind, whereas in true multicellular animals there are different kindsof cells, showing division of labour. There are some other Protozoa inwhich the nucleus or kernel divides into many nuclei within the cell.This is seen in the Giant Amoeba (Pelomyxa), sometimes found induck-ponds, or the beautiful Opalina, which always lives in the hindpart of the frog’s food-canal. If a portion of the living matter ofthese Protozoa should gather round each of the nuclei, then _that wouldbe the beginning of a body_. It would be still nearer the beginning of abody if division of labour set in, and if there was a setting apart ofegg-cells and sperm-cells distinct from body-cells.

It was possibly in some such way that animals and plants with a bodywere first evolved. Two points should be noticed, that body-making isnot essentially a matter of size, though it made large size possible.For the body of a many-celled Wheel Animalcule or Rotifer is no biggerthan many a Protozoon. Yet the Rotifer–we are thinking of Hydatina–hasnine hundred odd cells, whereas the Protozoon has only one, except informs like Volvox. Secondly, it is a luminous fact that _everymany-celled animal from sponge to man that multiplies in the ordinaryway begins at the beginning again as a “single cell,”_ the fertilisedegg-cell. It is, of course, not an ordinary single cell that developsinto an earthworm or a butterfly, an eagle, or a man; it is a cell inwhich a rich inheritance, the fruition of ages, is somehow condensed;but it is interesting to bear in mind the elementary fact that everymany-celled creature, reproduced in the ordinary way and not by buddingor the like, starts as a fertilised egg-cell. The coherence of thedaughter-cells into which the fertilised egg-cell divides is areminiscence, as it were, of the primeval coherence of daughter-unitsthat made the first body possible.

The Beginning of Sexual Reproduction

A freshwater Hydra, growing on the duckweed usually multiplies bybudding. It forms daughter-buds, living images of itself; a check comesto nutrition and these daughter-buds go free. A big sea-anemone maydivide in two or more parts, which become separate animals. This isasexual reproduction, which means that the multiplication takes place bydividing into two or many portions, and not by liberating egg-cells andsperm-cells. Among animals as among plants, asexual reproduction is verycommon. But it has great disadvantages, for it is apt to bephysiologically expensive, and it is beset with difficulties when thebody shows great division of labour, and is very intimately bound intounity. Thus, no one can think of a bee or a bird multiplying by divisionor by budding. Moreover, if the body of the parent has suffered frominjury or deterioration, the result of this is bound to be handed on tothe 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 arewell 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. Thestudy of these organisms has of late years acquired an immenseimportance on account of the widespread and dangerous maladies to whichsome of them give rise. It lives in the blood of man, who is infected bythe bite of a Tse-tse fly which carries the parasite from some otherhost.]

[Illustration: VOLVOX

The Volvox is found in some canals and the like. It is one of the firstanimals to suggest the beginning of a body. It is a colony of a thousandor even ten thousand cells, but they are all cells of one kind. In_multicellular_ animals the cells are of _different_ kinds withdifferent functions. Each of the ordinary cells (marked 5) has twolashes or flagella. Daughter colonies inside the Parent colony are beingformed 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 ofa body. There is a setting apart of egg-cells and sperm-cells, distinctfrom body-cells; the collared lashed cells on the margin are differentin kind from those farther in. Thus, as in indubitable multicellularanimals, division of labour has begun.]

Splitting into two or many parts was the old-fashioned way ofmultiplying, but one of the great steps in evolution was the discoveryof a better method, namely, sexual reproduction. The gist of this issimply that during the process of body-building (by the development ofthe fertilised egg-cell) certain units, _the germ-cells_, do not sharein forming ordinary tissues or organs, but remain apart, continuing thefull inheritance which was condensed in the fertilised egg-cell. _These

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