The Story of Evolution/Chapter VII

Slender as our knowledge is of the earlier evolution of the Invertebrate animals, we return to our Cambrian population with greater interest. The uncouth Trilobite and its livelier cousins, the sluggish, skulking Brachiopod and Mollusc, the squirming Annelids, and the plant-like Cystids, Corals, and Sponges are the outcome of millions of years of struggle. Just as men, when their culture and their warfare advanced, clothed themselves with armour, and the most completely mailed survived the battle, so, generation after generation, the thicker and harder-skinned animals survived in the Archaean battlefield, and the Cambrian age opened upon the various fashions of armour that we there described. But, although half the story of life is over, organisation is still imperfect and sluggish. We have now to see how it advances to higher levels, and how the drama is transferred from the ocean to a new and more stimulating environment.

The Cambrian age begins with a vigorous move on the part of the land. The seas roll back from the shores of the "lost Atlantis," and vast regions are laid bare to the sun and the rains. In the bays and hollows of the distant shores the animal survivors of the great upheaval adapt themselves to their fresh homes and continue the struggle. But the rivers and the waves are at work once more upon the land, and, as the Cambrian age proceeds, the fringes of the continents are sheared, and the shore-life steadily advances upon the low-lying land. By the end of the Cambrian age a very large proportion of the land is covered with a shallow sea, in which the debris of its surface is deposited. The levelling continues through the next (Ordovician) period. Before its close nearly the whole of the United States and the greater part of Canada are under water, and the new land that had appeared on the site of Europe is also for the most part submerged. The present British Isles are almost reduced to a strip of north-eastern Ireland, the northern extremity of Scotland, and large islands in the south-west and centre of England.

We have already seen that these victories of the sea are just as stimulating, in a different way, to animals as the victories of the land. American geologists are tracing, in a very instructive way, the effect on that early population of the encroachment of the sea. In each arm of the sea is a distinctive fauna. Life is still very parochial; the great cosmopolitans, the fishes, have not yet arrived. As the land is revelled, the arms of the sea approach each other, and at last mingle their waters and their populations, with stimulating effect. Provincial characters are modified, and cosmopolitan characters increase in the great central sea of America. The vast shallow waters provide a greatly enlarged theatre for the life of the time, and it flourishes enormously. Then, at the end of the Ordovician, the land begins to rise once more. Whether it was due to a fresh shrinking of the crust, or to the simple process we have described, or both, we need not attempt to determine; but both in Europe and America there is a great emergence of land. The shore-tracts and the shallow water are narrowed, the struggle is intensified in them, and we pass into the Silurian age with a greatly reduced number but more advanced variety of animals. In the Silurian age the sea advances once more, and the shore-waters expand. There is another great "expansive evolution" of life. But the Silurian age closes with a fresh and very extensive emergence of the land, and this time it will have the most important consequences. For two new things have meantime appeared on the earth. The fish has evolved in the waters, and the plant, at least, has found a footing on the land.

These geological changes which we have summarised and which have been too little noticed until recently in evolutionary studies, occupied 7,000,000 years, on the lowest estimate, and probably twice that period. The impatient critic of evolutionary hypotheses is apt to forget the length of these early periods. We shall see that in the last two or three million years of the earth's story most extraordinary progress has been made in plant and animal development, and can be very fairly traced. How much advance should we allow for these seven or fourteen million years of swarming life and changing environments?

We cannot nearly cover the whole ground of paleontology for the period, and must be content to notice some of the more interesting advances, and then deal more fully with the evolution of the fish, the forerunner of the great land animals.

The Trilobite was the most arresting figure in the Cambrian sea, and its fortunes deserve a paragraph. It reaches its climax in the Ordovician sea, and then begins to decline, as more powerful animals come upon the scene. At first (apparently) an eyeless organism, it gradually develops compound eyes, and in some species the experts have calculated that there were 15,000 facets to each eye. As time goes on, also, the eye stands out from the head on a kind of stalk, giving a wider range of vision. Some of the more sluggish species seem to have been able to roll themselves up, like hedgehogs, in their shells, when an enemy approached. But another branch of the same group (Crustacea) has meantime advanced, and it gradually supersedes the dwindling Trilobites. Toward the close of the Silurian great scorpion-like Crustaceans (Pterygotus, Eurypterus, etc.) make their appearance. Their development is obscure, but it must be remembered that the rocks only give the record of shore-life, and only a part of that is as yet opened by geology. Some experts think that they were developed in inland waters. Reaching sometimes a length of five or six feet, with two large compound eyes and some smaller eye-spots (ocelli), they must have been the giants of the Silurian ocean until the great sharks and other fishes appeared.

The quaint stalked Echinoderm which also we noticed in the Cambrian shallows has now evolved into a more handsome creature, the sea-lily. The cup-shaped body is now composed of a large number of limy plates, clothed with flesh; the arms are long, tapering, symmetrical, and richly fringed; the stalk advances higher and higher, until the flower-like animal sometimes waves its feathery arms from the top of a flexible pedestal composed of millions of tiny chalk disks. Small forests of these sea-lilies adorn the floor of the Silurian ocean, and their broken and dead frames form whole beds of limestone. The primitive Cystids dwindle and die out in the presence of such powerful competitors. Of 250 species only a dozen linger in the Silurian strata, though a new and more advanced type—the Blastoid—holds the field for a time. It is the age of the Crinoids or sea-lilies. The starfish, which has abandoned the stalk, does not seem to prosper as yet, and the brittle-star appears. Their age will come later. No sea-urchins or sea-cucumbers (which would hardly be preserved) are found as yet. It is precisely the order of appearance which our theory of their evolution demands.

The Brachiopods have passed into entirely new and more advanced species in the many advances and retreats of the shores, but the Molluscs show more interesting progress. The commanding group from the start is that of the Molluscs which have "kept their head," the Cephalopods, and their large shells show a most instructive evolution. The first great representative of the tribe is a straight-shelled Cephalopod, which becomes "the tyrant and scavenger of the Silurian ocean" (Chamberlin). Its tapering, conical shell sometimes runs to a length of fifteen feet, and a diameter of one foot. It would of itself be an important evolutionary factor in the primitive seas, and might explain more than one advance in protective armour or retreat into heavy shells. As the period advances the shell begins to curve, and at last it forms a close spiral coil. This would be so great an advantage that we are not surprised to find the coiled type (Goniatites) gain upon and gradually replace the straight-shelled types (Orthoceratites). The Silurian ocean swarms with these great shelled Cephalopods, of which the little Nautilus is now the only survivor.

We will not enlarge on the Sponges and Corals, which are slowly advancing toward the higher modern types. Two new and very powerful organisms have appeared, and merit the closest attention. One is the fish, the remote ancestor of the birds and mammals that will one day rule the earth. The other may be the ancestor of the fish itself, or it may be one of the many abortive outcomes and unsuccessful experiments of the stirring life of the time. And while these new types are themselves a result of the great and stimulating changes which we have reviewed and the incessant struggle for food and safety, they in turn enormously quicken the pace of development. The Dreadnought appears in the primitive seas; the effect on the fleets of the world of the evolution of our latest type of battleship gives us a faint idea of the effect, on all the moving population, of the coming of these monsters of the deep. The age had not lacked incentives to progress; it now obtains a more terrible and far-reaching stimulus.

To understand the situation let us see how the battle of land and sea had proceeded. The Devonian Period had opened with a fresh emergence of the land, especially in Europe, and great inland seas or lakes were left in the hollows. The tincture of iron which gives a red colour to our characteristic Devonian rocks, the Old Red Sandstone, shows us that the sand was deposited in inland waters. The fish had already been developed, and the Devonian rocks show it swarming, in great numbers and variety, in the enclosed seas and round the fringe of the continents.

The first generation was a group of strange creatures, half fish and half Crustacean, which are known as the Ostracoderms. They had large armour-plated heads, which recall the Trilobite, and suggest that they too burrowed in the mud of the sea or (as many think) of the inland lakes, making havoc among the shell-fish, worms, and small Crustacea. The hind-part of their bodies was remarkably fish-like in structure. But they had no backbone—though we cannot say whether they may not have had a rod of cartilage along the back—and no articulated jaws like the fish. Some regard them as a connecting link between the Crustacea and the fishes, but the general feeling is that they were an abortive development in the direction of the fish. The sharks and other large fishes, which have appeared in the Silurian, easily displace these clumsy and poor-mouthed competitors One almost thinks of the aeroplane superseding the navigable balloon.

Of the fishes the Arthrodirans dominated the inland seas (apparently), while the sharks commanded the ocean. One of the Arthrodirans, the Dinichthys ("terrible fish"), is the most formidable fish known to science. It measured twenty feet from snout to tail. Its monstrous head, three feet in width, was heavily armoured, and, instead of teeth, its great jaws, two feet in length, were sharpened, and closed over the victim like a gigantic pair of clippers. The strongly plated heads of these fishes were commonly a foot or two feet in width. Life in the waters became more exacting than ever. But the Arthrodirans were unwieldy and sluggish, and had to give way before more progressive types. The toothed shark gradually became the lord of the waters.

The early shark ate, amongst other things, quantities of Molluscs and Brachiopods. Possibly he began with Crustacea; in any case the practice of crunching shellfish led to a stronger and stronger development of the hard plate which lined his mouth. The prickles of the plate grew larger and harder, until—as may be seen to-day in the mouth of a young shark—the cavity was lined with teeth. In the bulk of the Devonian sharks these developed into what are significantly called "pavement teeth." They were solid plates of enamel, an inch or an inch and a half in width, with which the monster ground its enormous meals of Molluscs, Crustacea, sea-weed, etc. A new and stimulating element had come into the life of the invertebrate world. Other sharks snapped larger victims, and developed the teeth on the edges of their jaws, to the sacrifice of the others, until we find these teeth in the course of time solid triangular masses of enamel, four or five inches long, with saw-like edges. Imagine these terrible mouths—the shears of the Arthrodiran, and the grindstones and terrible crescents of the giant sharks—moving speedily amongst the crowded inhabitants of the waters, and it is easy to see what a stimulus to the attainment of speed and of protective devices was given to the whole world of the time.

What was the origin of the fish? Here we are in much the same position as we were in regard to the origin of the higher Invertebrates. Once the fish plainly appears upon the scene it is found to be undergoing a process of evolution like all other animals. The vast majority of our fishes have bony frames (or are Teleosts); the fishes of the Devonian age nearly all have frames of cartilage, and we know from embryonic development that cartilage is the first stage in the formation of bone. In the teeth and tails, also, we find a gradual evolution toward the higher types. But the earlier record is, for reasons I have already given, obscure; and as my purpose is rather to discover the agencies of evolution than to strain slender evidence in drawing up pedigrees, I need only make brief reference to the state of the problem.

Until comparatively recent times the animal world fell into two clearly distinct halves, the Vertebrates and the Invertebrates. There were several anatomical differences between the two provinces, but the most conspicuous and most puzzling was the backbone. Nowhere in living nature or in the rocks was any intermediate type known between the backboned and the non-backboned animal. In the course of the nineteenth century, however, several animals of an intermediate type were found. The sea-squirt has in its early youth the line of cartilage through the body which, in embryonic development, represents the first stage of the backbone; the lancelet and the Appendicularia have a rod of cartilage throughout life; the "acorn-headed worm" shows traces of it. These are regarded as surviving specimens of various groups of animals which, in early times, fell between the Invertebrate and Vertebrate worlds, and illustrate the transition.

With their aid a genealogical tree was constructed for the fish. It was assumed that some Cambrian or Silurian Annelid obtained this stiffening rod of cartilage. The next advantage—we have seen it in many cases—was to combine flexibility with support. The rod was divided into connected sections (vertebrae), and hardened into bone. Besides stiffening the body, it provided a valuable shelter for the spinal cord, and its upper part expanded into a box to enclose the brain. The fins were formed of folds of skin which were thrown off at the sides and on the back, as the animal wriggled through the water. They were of use in swimming, and sections of them were stiffened with rods of cartilage, and became the pairs of fins. Gill slits (as in some of the highest worms) appeared in the throat, the mouth was improved by the formation of jaws, and—the worm culminated in the shark.

Some experts think, however, that the fish developed directly from a Crustacean, and hold that the Ostracoderms are the connecting link. A close discussion of the anatomical details would be out of place here, and the question remains open for the present. Directly or indirectly, the fish is a descendant of some Archaean Annelid. It is most probable that the shark was the first true fish-type. There are unrecognisable fragments of fishes in the Ordovician and Silurian rocks, but the first complete skeletons (Lanarkia, etc.) are of small shark- like creatures, and the low organisation of the group to which the shark belongs, the Elasmobranchs, makes it probable that they are the most primitive. Other remains (Palaeospondylus) show that the fish-like lampreys had already developed.

Two groups were developed from the primitive fish, which have great interest for us. Our next step, in fact, is to trace the passage of the fish from the water to the land, one of the most momentous chapters in the story of life. To that incident or accident of primitive life we owe our own existence and the whole development of the higher types of animals. The advance of natural history in modern times has made this passage to the land easy to understand. Not only does every frog reenact it in the course of its development, but we know many fishes that can live out of water. There is an Indian perch—called the "climbing perch," but it has only once been seen by a European to climb a tree—which crosses the fields in search of another pool, when its own pool is evaporating. An Indian marine fish (Periophthalmus) remains hunting on the shore when the tide goes out. More important still, several fishes have lungs as well as gills. The Ceratodus of certain Queensland rivers has one lung; though, I was told by the experts in Queensland, it is not a "mud-fish," and never lives in dry mud. However, the Protopterus of Africa and the Lepidosiren of South America have two lungs, as well as gills, and can live either in water or, in the dry season, on land.

When the skeletons of fishes of the Ceratodus type were discovered in the Devonian rocks, it was felt that we had found the fish-ancestor of the land Vertebrates, but a closer anatomical examination has made this doubtful. The Devonian lung-fish has characters which do not seem to lead on to the Amphibia. The same general cause probably led many groups to leave the water, or adapt themselves to living on land as well as in water, and the abundant Dipoi or Dipneusts ("double-breathers") of the Devonian lakes are one of the chief of these groups, which have luckily left descendants to our time. The ancestors of the Amphibia are generally sought amongst the Crossopterygii, a very large group of fishes in Devonian times, with very few representatives to-day.

It is more profitable to investigate the process itself than to make a precarious search for the actual fish, and, fortunately, this inquiry is more hopeful. The remains that we find make it probable that the fish left the water about the beginning of the Devonian or the end of the Silurian. Now this period coincides with two circumstances which throw a complete light on the step; one is the great rise of the land, catching myriads of fishes in enclosed inland seas, and the other is the appearance of formidable carnivores in the waters. As the seas evaporated and the great carnage proceeded, the land, which was already covered with plants and inhabited by insects, offered a safe retreat for such as could adopt it. Emigration to the land had been going on for ages, as we shall see. Curious as it must seem to the inexpert, the fishes, or some of them, were better prepared than most other animals to leave the water. The chief requirement was a lung, or interior bag, by which the air could be brought into close contact with the absorbing blood vessels. Such a bag, broadly speaking, most of the fishes possess in their floating-bladder: a bag of gas, by compressing or expanding which they alter their specific gravity in the water. In some fishes it is double; in some it is supplied with blood-vessels; in some it is connected by a tube with the gullet, and therefore with the atmosphere.

Thus we get very clear suggestions of the transition from water to land. We must, of course, conceive it as a slow and gradual adaptation. At first there may have been a rough contrivance for deriving oxygen directly and partially from the atmosphere, as the water of the lake became impure. So important an advantage would be fostered, and, as the inland sea became smaller, or its population larger or fiercer, the fishes with a sufficiently developed air-breathing apparatus passed to the land, where, as yet, they would find no serious enemy. The fact is beyond dispute; the theory of how it occurred is plausible enough; the consequences were momentous. Great changes were preparing on the land, and in a comparatively short time we shall find its new inhabitant subjected to a fierce test of circumstances that will carry it to an enormously higher level than life had yet reached.

I have said that the fact of this transition to the land is beyond dispute. The evidence is very varied, but need not all be enlarged upon here. The widespread Dipneust fishes of the Devonian rocks bear strong witness to it, and the appearance of the Amphibian immediately afterwards makes it certain. The development of the frog is a reminiscence of it, on the lines of the embryonic law which we saw earlier. An animal, in its individual development, more or less reproduces the past phases of its ancestry. So the free-swimming jelly-fish begins life as a fixed polyp; a kind of star-fish (Comatula) opens its career as a stalked sea-lily; the gorgeous dragon-fly is at first an uncouth aquatic animal, and the ethereal butterfly a worm-like creature. But the most singular and instructive of all these embryonic reminiscences of the past is found in the fact that all the higher land-animals of to-day clearly reproduce a fish-stage in their embryonic development.

In the third and fourth weeks of development the human embryo shows four (closed) slits under the head, with corresponding arches. The bird, the dog, the horse—all the higher land animals, in a word, pass through the same phase. The suggestion has been made that these structures do not recall the gill-slits and gill-arches of the fish, but are folds due to the packing of the embryo in the womb. In point of fact, they appear just at the time when the human embryo is only a fifth of an inch long, and there is no such compression. But all doubt as to their interpretation is dispelled when we remove the skin and examine the heart and blood-vessels. The heart is up in the throat, as in the fish, and has only two chambers, as in the fish (not four, as in the bird and mammal); and the arteries rise in five pairs of arches over the swellings in the throat, as they do in the lower fish, but do not in the bird and mammal. The arrangement is purely temporary—lasting only a couple of weeks in the human embryo—and purposeless. Half these arteries will disappear again. They quite plainly exist to supply fine blood-vessels for breathing at the gill-clefts, and are never used, for the embryo does not breathe, except through the mother. They are a most instructive reminder of the Devonian fish which quitted its element and became the ancestor of all the birds and mammals of a later age.

Several other features of man's embryonic development—the budding of the hind limbs high up, instead of at the base of, the vertebral column, the development of the ears, the nose, the jaws, etc.—have the same lesson, but the one detailed illustration will suffice. The millions of years of stimulating change and struggle which we have summarised have resulted in the production of a fish which walks on four limbs (as the South American mud-fish does to-day), and breathes the atmosphere.

We have been quite unable to follow the vast changes which have meantime taken place in its organisation. The eyes, which were mere pits in the skin, lined with pigment cells, in the early worm, now have a crystalline lens to concentrate the light and define objects on the nerve. The ears, which were at first similar sensitive pits in the skin, on which lay a little stone whose movements gave the animal some sense of direction, are now closed vesicles in the skull, and begin to be sensitive to waves of sound. The nose, which was at first two blind, sensitive pits in the skin of the head, now consists of two nostrils opening into the mouth, with an olfactory nerve spreading richly over the passages. The brain, which was a mere clump of nerve-cells connecting the rough sense-impressions, is now a large and intricate structure, and already exhibits a little of that important region (the cerebrum) in which the varied images of the outside world are combined. The heart, which was formerly was a mere swelling of a part of one of the blood-vessels, now has two chambers.

We cannot pursue these detailed improvements of the mechanism, as we might, through the ascending types of animals. Enough if we see more or less clearly how the changes in the face of the earth and the rise of its successive dynasties of carnivores have stimulated living things to higher and higher levels in the primitive ocean. We pass to the clearer and far more important story of life on land, pursuing the fish through its continuous adaptations to new conditions until, throwing out side-branches as it progresses, it reaches the height of bird and mammal life.