# The Story of Evolution/Chapter XVII

In our study of the evolution of the plant, the insect, and the bird we were seriously thwarted by the circumstance that their frames, somewhat frail in themselves, were rarely likely to be entombed in good conditions for preservation. Earlier critics of evolution used, when they were imperfectly acquainted with the conditions of fossilisation, to insinuate that this fragmentary nature of the geological record was a very convenient refuge for the evolutionist who was pressed for positive evidence. The complaint is no longer found in any serious work. Where we find excellent conditions for preservation, and animals suitable for preservation living in the midst of them, the record is quite satisfactory. We saw how the chalk has yielded remains of sea-urchins in the actual and gradual process of evolution. Tertiary beds which represent the muddy bottoms of tranquil lakes are sometimes equally instructive in their fossils, especially of shell-fish. The Paludina of a certain Slavonian lake-deposit is a classical example. It changes so greatly in the successive levels of the deposit that, if the intermediate forms were not preserved, we should divide it into several different species. The Planorbis is another well-known example. In this case we have a species evolving along several distinct lines into forms which differ remarkably from each other.
The Tertiary mammals, living generally on the land and only coming by accident into deposits suitable for preservation, cannot be expected to reveal anything like this sensible advance from form to form. They were, however, so numerous in the mid-Tertiary, and their bones are so well calculated to survive when they do fall into suitable conditions, that we can follow their development much more easily than that of the birds. We find a number of strange patriarchal beasts entering the scene in the early Eocene, and spreading into a great variety of forms in the genial conditions of the Oligocene and Miocene. As some of these forms advance, we begin to descry in them the features, remote and shadowy at first, of the horse, the deer, the elephant, the whale, the tiger, and our other familiar mammals. In some instances we can trace the evolution with a wonderful fullness, considering the remoteness of the period and the conditions of preservation. Then, one by one, the abortive, the inelastic, the ill-fitted types are destroyed by changing conditions or powerful carnivores, and the field is left to the mammals which filled it when man in turn began his destructive career.
The first point of interest is the origin of these Tertiary mammals. Their distinctive advantage over the mammals of the Mesozoic Era was-the possession by the mother of a placenta (the "after-birth" of the higher mammals), or structure in the womb by which the blood-vessels of the mother are brought into such association with those of the foetus that her blood passes into its arteries, and it is fully developed within the warm shelter of her womb. The mammals of the Mesozoic had been small and primitive animals, rarely larger than a rat, and never rising above the marsupial stage in organisation. They not only continued to exist, and give rise to their modern representatives (the opossum, etc.) during the Tertiary Era, but they shared the general prosperity. In Australia, where they were protected from the higher carnivorous mammals, they gave rise to huge elephant-like wombats (Diprotodon), with skulls two or three feet in length. Over the earth generally, however, they were superseded by the placental mammals, which suddenly break into the geological record in the early Tertiary, and spread with great vigour and rapidity over the four continents.
Were they a progressive offshoot from the Mesozoic Marsupials, or Monotremes, or do they represent a separate stock from the primitive half-reptile and half-mammal family? The point is disputed; nor does the scantiness of the record permit us to tell the place of their origin. The placental structure would be so great an advantage in a cold and unfavourable environment that some writers look to the northern land, connecting Europe and America, for their development. We saw, however, that this northern region was singularly warm until long after the spread of the mammals. Other experts, impressed by the parallel development of the mammals and the flowering plants, look to the elevated parts of eastern North America.
Such evidence as there is seems rather to suggest that South Africa was the cradle of the placental mammals. We shall find that many of our mammals originated in Africa; there, too, is found to-day the most primitive representative of the Tertiary mammals, the hyrax; and there we find in especial abundance the remains of the mammal-like reptiles (Theromorphs) which are regarded as their progenitors. Further search in the unexplored geological treasures and dense forests of Africa is needed. We may provisionally conceive the placental mammals as a group of the South African early mammals which developed a fortunate variation in womb-structure during the severe conditions of the early Mesozoic. In this new structure they would have no preponderant advantage as long as the genial Jurassic age favoured the great reptiles, and they may have remained as small and insignificant as the Marsupials. But with the fresh upheaval and climatic disturbance at the end of the Jurassic, and during the Cretaceous, they spread northward, and replaced the dying reptiles, as the Angiosperms replaced the dying cycads. When they met the spread of the Angiosperm vegetation they would receive another great stimulus to development.
They appear in Europe and North America in the earliest Cretaceous. The rise of the land had connected many hitherto isolated regions, and they seem to have poured over every bridge into all parts of the four continents. The obscurity of their origin is richly compensated by their intense evolutionary interest from the moment they enter the geological record. We have seen this in the case of every important group of plants and animals, and can easily understand it. The ancestral group was small and local; the descendants are widely spread. While, therefore, we discover remains of the later phases of development in our casual cuttings and quarries, the ancestral tomb may remain for ages in some unexplored province of the geological world. If this region is, as we suspect, in Africa, our failure to discover it as yet is all the more intelligible.
But these mammals of the early Tertiary are still of such a patriarchal or ancestral character that the student of evolution can dispense with their earlier phase. They combine in their primitive frames, in an elementary way, the features which we now find distributed in widely removed groups of their descendants. Most of them fall into two large orders: the Condylarthra, the ancestral herbivores from which we shall find our horses, oxen, deer, elephants, and hogs gradually issuing, and the Creodonta, the patriarchal carnivores, which will give birth to our lions and tigers, wolves and foxes, and their various cousins. As yet even the two general types of herbivore and carnivore are so imperfectly separated that it is not always possible to distinguish between them. Nearly all of them have the five-toed foot of the reptile ancestor; and the flat nails on their toes are the common material out of which the hoof of the ungulate and the claw of the carnivore will be presently fashioned. Nearly all have forty-four simply constructed teeth, from which will be evolved the grinders and tusks of the elephant or the canines of the tiger. They answer in every respect to the theory that some primitive local group was the common source of all our great mammals. With them are ancestral forms of Edentates (sloths, etc.) and Insectivores (moles, etc.), side-branches developing according to their special habits; and before the end of the Eocene we find primitive Rodents (squirrels, etc.) and Cheiroptera (bats).
From the description of the Tertiary world which we have seen in the last chapter we understand the rapid evolution of the herbivorous Condylarthra. The rich vegetation which spreads over the northern continents, to which they have penetrated, gives them an enormous vitality and fecundity, and they break into groups, as they increase in number, adapted to the different conditions of forest, marsh, or grass-covered plain. Some of them, swelling lazily on the abundant food, and secure for a time in their strength, become the Deinosaurs of their age, mere feeding and breeding machines. They are massive, sluggish, small-brained animals, their strong stumpy limbs terminating in broad five-toed feet. Coryphodon, sometimes as large as an ox, is a typical representative. It is a type fitted only for prosperous days, and these Amblypoda, as they are called, will disappear as soon as the great carnivores are developed.
Another doomed race, or abortive experiment of early mammal life, were the remarkable Deinocerata ("terrible-horned" mammals). They sometimes measured thirteen feet in length, but had little use for brain in the conditions in which they were developed. The brain of the Deinoceras was only one-eighth the size of the brain of a rhinoceros of the same bulk; and the rhinoceros is a poor-brained representative of the modern mammals. To meet the growing perils of their race they seem to have developed three pairs of horns on their long, flat skulls, as we find on them three pairs of protuberances. A late specimen of the group, Tinoceras, had a head four feet in length, armed with these six horns, and its canine teeth were developed into tusks sometimes seven or eight inches in length. They suggest a race of powerful but clumsy and grotesque monsters, making a last stand, and developing such means of protection as their inelastic nature permitted. But the horns seem to have proved a futile protection against the advancing carnivores, and the race was extinguished. The horns may, of course, have been mainly developed by, or for, the mutual butting of the males.
The extinction of these races will remind many readers of a theory on which it is advisable to say a word. It will be remembered that the last of the Deinosaurs and the Ammonites also exhibited some remarkable developments in their last days. These facts have suggested to some writers the idea that expiring races pass through a death-agony, and seem to die a natural death of old age like individuals. The Trilobites are quoted as another instance; and some ingenious writers add the supposed eccentricities of the Roman Empire in its senile decay and a number of other equally unsubstantial illustrations.
There is not the least ground for this fantastic speculation. The destruction of these "doomed races" is as clearly traceable to external causes as is the destruction of the Roman Empire; nor, in fact, did the Roman Empire develop any such eccentricities as are imagined in this superficial theory. What seem to our eye the "eccentricities" and "convulsions" of the Ceratopsia and Deinocerata are much more likely to be defensive developments against a growing peril, but they were as futile against the new carnivores as were the assegais of the Zulus against the European. On the other hand, the eccentricities of many of the later Trilobites—the LATEST Trilobites, it may be noted, were chaste and sober specimens of their race, like the last Roman patricians—and of the Ammonites may very well have been caused by physical and chemical changes in the sea-water. We know from experiment that such changes have a disturbing influence, especially on the development of eggs and larvae; and we know from the geological record that such changes occurred in the periods when the Trilobites and Ammonites perished. In fine, the vast majority of extinct races passed through no "convulsions" whatever. We may conclude that races do not die; they are killed.
The extinction of these races of the early Condylarthra, and the survival of those races whose descendants share the earth with us to-day, are quite intelligible. The hand of natural selection lay heavy on the Tertiary herbivores. Apart from overpopulation, forcing groups to adapt themselves to different regions and diets, and apart from the geological disturbances and climatic changes which occurred in nearly every period, the shadow of the advancing carnivores was upon them. Primitive but formidable tigers, wolves, and hyenas were multiplying, and a great selective struggle set in. Some groups shrank from the battle by burrowing underground like the rabbit; some, like the squirrel or the ape, took refuge in the trees; some, like the whale and seal, returned to the water; some shrank into armour, like the armadillo, or behind fences of spines, like the hedgehog; some, like the bat, escaped into the air. Social life also was probably developed at this time, and the great herds had their sentinels and leaders. But the most useful qualities of the large vegetarians, which lived on grass and leaf, were acuteness of perception to see the danger, and speed of limb to escape it. In other words, increase of brain and sense-power and increase of speed were the primary requisites. The clumsy early Condylarthra failed to meet the tests, and perished; the other branches of the race were more plastic, and, under the pressure of a formidable enemy, were gradually moulded into the horse, the deer, the ox, the antelope, and the elephant.
We can follow the evolution of our mammals of this branch most easily by studying the modification of the feet and limbs. In a running attitude—the experiment may be tried—the weight of the body is shifted from the flat sole of the foot, and thrown upon the toes, especially the central toes. This indicates the line of development of the Ungulates (hoofed animals) in the struggle of the Tertiary Era. In the early Eocene we find the Condylarthra (such as Phenacodus) with flat five-toed feet, and such a mixed combination of characters that they "might serve very well for the ancestors of all the later Ungulata" (Woodward). We then presently find this generalised Ungulate branching into three types, one of which seems to be a patriarchal tapir, the second is regarded as a very remote ancestor of the horse, and the third foreshadows the rhinoceros. The feet have now only three or four toes; one or two of the side-toes have disappeared. This evolution, however, follows two distinct lines. In one group of these primitive Ungulates the main axis of the limb, or the stress of the weight, passes through the middle toe. This group becomes the Perissodactyla ("odd-toed" Ungulates) of the zoologist, throwing out side-branches in the tapir and the rhinoceros, and culminating in the one-toed horse. In the other line, the Artiodactyla (the "even-toed" or cloven-hoofed Ungulates), the main axis or stress passes between the third and fourth toes, and the group branches into our deer, oxen, sheep, pigs, camels, giraffes, and hippopotamuses. The elephant has developed along a separate and very distinctive line, as we shall see, and the hyrax is a primitive survivor of the ancestral group.
Thus the evolutionist is able to trace a very natural order in the immense variety of our Ungulates. He can follow them in theory as they slowly evolve from their primitive Eocene ancestor according to their various habits and environments; he has a very rich collection of fossil remains illustrating the stages of their development; and in the hyrax (or "coney") he has one more of those living fossils, or primitive survivors, which still fairly preserve the ancestral form. The hyrax has four toes on the front foot and three on the hind foot, and the feet are flat. Its front teeth resemble those of a rodent, and its molars those of the rhinoceros. In many respects it is a most primitive and generalised little animal, preserving the ancestral form more or less faithfully since Tertiary days in the shelter of the African Continent.
The rest of the Ungulates continued to develop through the Tertiary, and fortunately we are enabled to follow the development of two of the most interesting of them, the horse and the elephant, in considerable detail. As I said above, the primitive Ungulate soon branches into three types which dimly foreshadow the tapir, the horse, and the rhinoceros, the three forms of the Perissodactyl. The second of these types is the Hyracotherium. It has no distinct equine features, and is known only from the skull, but the authorities regard it as the progenitor (or representative of the progenitors) of the horse-types. In size it must have been something like the rabbit or the hyrax. Still early in the Eocene, however, we find the remains of a small animal (Eohippus), about the size of a fox, which is described as "undoubtedly horse-like." It had only three toes on its hind feet, and four on its front feet; though it had also a splint-bone, representing the shrunken and discarded fifth toe, on its fore feet. Another form of the same period (Protorohippus) shows the central of the three toes on the hind foot much enlarged, and the lateral toes shrinking. The teeth, and the bones and joints of the limbs, are also developing in the direction of the horse.
In the succeeding geological period, the Oligocene, we find several horse-types in which the adaptation of the limbs to running on the firm grassy plains and of the teeth to eating the grass continues. Mesohippus has lost the fourth toe of the fore foot, which is now reduced to a splintbone, and the lateral toes of its hind foot are shrinking. In the Miocene period there is a great development of the horse-like mammals. We have the remains of more than forty species, some continuing the main line of development on the firm and growing prairies of the Miocene, some branching into the softer meadows or the forests, and giving rise to types which will not outlive the Tertiary. They have three toes on each foot, and have generally lost even the rudimentary trace of the fourth toe. In most of them, moreover, the lateral toes—except in the marsh-dwelling species, with spreading feet—scarcely touch the ground, while the central toe is developing a strong hoof. The leg-bones are longer, and have a new type of joint; the muscles are concentrated near the body. The front teeth are now chopping incisors, and the grinding teeth approach those of the modern horse in the distribution of the enamel, dentine, and cement. They are now about the size of a donkey, and must have had a distinctly horsy appearance, with their long necks and heads and tapering limbs. One of them, Merychippus, was probably in the direct line of the evolution of the horse. From Hipparion some of the authorities believe that the zebras may have been developed. Miohippus, Protohippus, and Hypohippus, varying in size from that of a sheep to that of a donkey, are other branches of this spreading family.
In the Pliocene period the evolution of the main stem culminates in the appearance of the horse, and the collateral branches are destroyed. Pliohippus is a further intermediate form. It has only one toe on each foot, with two large splint bones, but its hoof is less round than that of the horse, and it differs in the shape of the skull and the length of the teeth. The true horse (Equus) at length appears, in Europe and America, before the close of the Tertiary period. As is well known, it still has the rudimentary traces of its second and fourth toes in the shape of splint bones, and these bones are not only more definitely toe-shaped in the foal before birth, but are occasionally developed and give us a three-toed horse.
From these successive remains we can confidently picture the evolution, during two or three million years, of one of our most familiar mammals. It must not, of course, be supposed that these fossil remains all represent "ancestors of the horse." In some cases they may very well do so; in others, as we saw, they represent sidebranches of the family which have become extinct. But even such successive forms as the Eohippus, Mesohippus, Miohippus, and Pliohippus must not be arranged in a direct line as the pedigree of the horse. The family became most extensive in the Miocene, and we must regard the casual fossil specimens we have discovered as illustrations of the various phases in the development of the horse from the primitive Ungulate. When we recollect what we saw in an earlier chapter about the evolution of grassy plains and the successive rises of the land during the Tertiary period, and when we reflect on the simultaneous advance of the carnivores, we can without difficulty realise this evolution of our familiar companion from a hyrax-like little animal of two million years ago.
We have not in many cases so rich a collection of intermediate forms as in the case of the horse, but our fossil mammals are numerous enough to suggest a similar development of all the mammals of to-day. The primitive family which gave birth to the horse also gave us, as we saw, the tapir and the rhinoceros. We find ancestral tapirs in Europe and America during the Tertiary period, but the later cold has driven them to the warm swamps of Brazil and Malaysia. The rhinoceros has had a long and interesting history. From the primitive Hyrochinus of the Eocene, in which it is dimly foreshadowed, we pass to a large and varied family in the later periods. In the Oligocene it spreads into three great branches, adapted, respectively, to life on the elevated lands, the lowlands, and the water. The upland type (Hyracodon) was a light-limbed running animal, well illustrating the close relation to the horse. The aquatic representative (Metamynodon) was a stumpy and bulky animal. The intermediate lowland type was probably the ancestor of the modern animal. All three forms were yet hornless. In the Miocene the lowland type (Leptaceratherium, Aceratherium, etc.) develops vigorously, while the other branches die. The European types now have two horns, and in one of the American species (Diceratherium) we see a commencement of the horny growths from the skull. We shall see later that the rhinoceros continued in Europe even during the severe conditions of the glacial period, in a branch that developed a woolly coat.
There were also in the early Tertiary several sidebranches of the horse-tapir-rhinoceros family. The Palaeotheres were more or less between the horse and the tapir in structure; the Anoplotheres between the tapir and the ruminant. A third doomed branch, the Titanotheres, flourished vigorously for a time, and begot some strange and monstrous forms (Brontops, Titanops, etc.). In the larger specimens the body was about fourteen feet long, and stood ten feet from the ground. The long, low skull had a pair of horns over the snout. They perished like the equally powerful but equally sluggish and stupid Deinocerata. The Tertiary was an age of brain rather than of brawn. As compared with their early Tertiary representatives' some of our modern mammals have increased seven or eight-fold in brain-capacity.
While the horses and tapirs and rhinoceroses were being gradually evolved from the primitive types, the Artiodactyl branch of the Ungulates—the pigs, deer, oxen, etc.—were also developing. We must dismiss them briefly. We saw that the primitive herbivores divided early in the Eocene into the "odd-toed" and "even-toed" varieties; the name refers, it will be remembered, not to the number of toes, but to the axis of stress. The Artiodactyl group must have quickly branched in turn, as we find very primitive hogs and camels before the end of the Eocene. The first hog-like creature (Homacodon) was much smaller than the hog of to-day, and had strong canine teeth, but in the Oligocene the family gave rise to a large and numerous race, the Elotheres. These "giant-pigs," as they have been called, with two toes on each foot, flourished vigorously for a time in Europe and America, but were extinguished in the Miocene, when the true pigs made their appearance. Another doomed race of the time is represented by the Hyopotamus, an animal between the pig and the hippopotamus; and the Oreodontids, between the hog and the deer, were another unsuccessful branch of the early race. The hippopotamus itself was widespread in Europe, and a familiar form in the rivers of Britain, in the latter part of the Tertiary.
The camel seems to be traceable to a group of primitive North American Ungulates (Paebrotherium, etc.) in the later Eocene period. The Paebrotherium, a small animal about two feet long, is followed by Pliauchenia, which points toward the llamas and vicunas, and Procamelus, which clearly foreshadows the true camel. In the Pliocene the one branch went southward, to develop into the llamas and vicunas, and the other branch crossed to Asia, to develop into the camels. Since that time they have had no descendants in North America.
The primitive giraffe appears suddenly in the later Tertiary deposits of Europe and Asia. The evidence points to an invasion from Africa, and, as the region of development is unknown and unexplored, the evolution of the giraffe remains a matter of speculation. Chevrotains flourished in Europe and North America in the Oligocene, and are still very primitive in structure, combining features of the hog and the ruminants. Primitive deer and oxen begin in the Miocene, and seem to have an earlier representative in certain American animals (Protoceras), of which the male has a pair of blunt outgrowths between the ears. The first true deer are hornless (like the primitive muskdeer of Asia to-day), but by the middle of the Miocene the males have small two-pronged antlers, and as the period proceeds three or four more prongs are added. It is some confirmation of the evolutionary embryonic law that we find the antlers developing in this way in the individual stag to-day. A very curious race of ruminants in the later Tertiary was a large antelope (Sivatherium) with four horns. It had not only the dimensions, but apparently some of the characters, of an elephant.
The elephant itself, the last type of the Ungulates, has a clearer line of developments. A chance discovery of fossils in the Fayum district in Egypt led Dr. C. W. Andrews to make a special exploration, and on the remains which he found he has constructed a remarkable story of the evolution of the elephant. It is clear that the elephant was developed in Africa, and a sufficiently complete series of remains has been found to give a good idea of the origin of its most distinctive features. In the Eocene period there lived in the Egyptian region an animal, something like the tapir in size and appearance, which had its second incisors developed into small tusks and—to judge from the nasal opening in the skull—a somewhat prolonged snout. This animal (Moeritherium) only differed from the ordinary primitive Ungulate in these incipient elephantine features. In the later Eocene a larger and more advanced animal, the Palaeomastodon, makes its appearance. Its tusks are larger (five or six inches long), its molars more elephantine, the air-cells at the back of the head more developed. It would look like a small elephant, except that it had a long snout, instead of a flexible trunk, and a projecting lower jaw on which the snout rested.
Up to the beginning of the Miocene, Africa was, as we saw, cut off from Europe and Asia by the sea which stretched from Spain to India. Then the land rose, and the elephant passed by the new tracts into the north. Its next representative, Tetrabelodon, is found in Asia and Europe, as well as North Africa. The frame is as large as that of a medium-sized elephant, and the increase of the air-cells at the back of the skull shows that an increased weight has to be sustained by the muscles of the neck. The nostrils are shifted further back. The tusks are from twenty to thirty inches long, and round, and only differ from those of the elephant in curving slightly downward, The chin projects as far as the tusks. The neck is shorter and thicker, and, as the animal increases in height, we can understand that the long snout—possibly prehensile at its lower end—is necessary for the animal to reach the ground. But the snout still lies on the projecting lower jaw, and is not a trunk. Passing over the many collateral branches, which diverge in various directions, we next kind that the chin is shortening (in Tetrabelodon longirostris), and, through a long series of discovered intermediate forms, we trace the evolution of the elephant from the mastodon. The long supporting skin disappears, and the enormous snout becomes a flexible trunk. Southern Asia seems to have been the province of this final transformation, and we have remains of some of these primitive elephants with tusks nine and a half feet long. A later species, which wandered over Central and Southern Europe before the close of the Tertiary, stood fifteen feet high at the shoulder, while the mammoth, which superseded it in the days of early man, had at times tusks more than ten feet in length.
It is interesting to reflect that this light on the evolution of one of our most specialised mammals is due to the chance opening of the soil in an obscure African region. It suggests to us that as geological exploration is extended, many similar discoveries may be made. The slenderness of the geological record is a defect that the future may considerably modify.
From this summary review of the evolution of the Ungulates we must now pass to an even briefer account of the evolution of the Carnivores. The evidence is less abundant, but the characters of the Carnivores consist so obviously of adaptations to their habits and diet that we have little difficulty in imagining their evolution. Their early Eocene ancestors, the Creodonts, gave rise in the Eocene to forms which we may regard as the forerunners of the cat-family and dog-family, to which most of our familiar Carnivores belong. Patriofelis, the "patriarchal cat," about five or six feet in length (without the tail), curiously combines the features of the cat and the seal-family. Cyonodon has a wolf-like appearance, and Amphicyon rather suggests the fox. Primitive weasels, civets, and hyaenas appear also in the Eocene. The various branches of the Carnivore family are already roughly represented, but it is an age of close relationships and generalised characters.
In the Miocene we find the various groups diverging still further from each other and from the extinct stocks. Definite wolves and foxes abound in America, and the bear, civet, and hyaena are represented in Europe, together with vague otter-like forms. The dog-family seems to have developed chiefly in North America. As in the case of the Ungulates, we find many strange side-branches which flourished for a time, but are unknown to-day. Machoerodus, usually known as "the sabre-toothed tiger," though not a tiger, was one of the most formidable of these transitory races. Its upper canine teeth (the "sabres") were several inches in length, and it had enormously distensible jaws to make them effective. The great development of such animals, with large numbers of hyaenas, civets, wolves, bears, and other Carnivores, in the middle and later Tertiary was probably the most effective agency in the evolution of the horse and deer and the extinction of the more sluggish races. The aquatic branch of the Carnivores (seals, walruses, etc.) is little represented in the Tertiary record. We saw, however, that the most primitive representatives of the elephant-stock had also some characters of the seal, and it is thought that the two had a common origin.
The Moeritherium was a marsh-animal, and may very well have been cousin to the branch of the family which pushed on to the seas, and developed its fore limbs into paddles.
The Rodents are represented in primitive form early in the Eocene period. The teeth are just beginning to show the characteristic modification for gnawing. A large branch of the family, the Tillodonts, attained some importance a little later. They are described as combining the head and claws of a bear with the teeth of a rodent and the general characters of an ungulate. In the Oligocene we find primitive squirrels, beavers, rabbits, and mice. The Insectivores also developed some of the present types at an early date, and have since proved so unprogressive that some regard them as the stock from which all the placental mammals have arisen.
The Cetacea (whales, porpoises, etc.) are already represented in the Eocene by a primitive whale-like animal (Zeuglodon) of unknown origin. Some specimens of it are seventy feet in length. It has large teeth, sometimes six inches long, and is clearly a terrestrial mammal that has returned to the waters. Some forms even of the modern whale develop rudimentary teeth, and in all forms the bony structure of the fore limbs and degenerate relic of a pelvis and back limbs plainly tell of the terrestrial origin. Dolphins appear in the Miocene.
Finally, the Edentates (sloths, anteaters, and armadilloes) are represented in a very primitive form in the early Eocene. They are then barely distinguishable from the Condylarthra and Creodonta, and seem only recently to have issued from a common ancestor with those groups. In the course of the Tertiary we find them—especially in South America, which was cut off from the North and its invading Carnivores during the Eocene and Miocene—developed into large sloths, armadilloes, and anteaters. The reconnection with North America in the Pliocene allowed the northern animals to descend, but gigantic sloths (Megatherium) and armadilloes (Glyptodon) flourished long afterwards in South America. The Megatherium attained a length of eighteen feet in one specimen discovered, and the Glyptodon often had a dorsal shield (like that of the armadillo) from six to eight feet long, and, in addition, a stoutly armoured tail several feet long.
The richness and rapidity of the mammalian development in the Tertiary, of which this condensed survey will convey some impression, make it impossible to do more here than glance over the vast field and indicate the better-known connections. It will be seen that evolution not only introduces a lucid order and arrangement into our thousands of species of living and fossil mammals, but throws an admirable light on the higher animal world of our time. The various orders into which the zoologist puts our mammals are seen to be the branches of a living tree, approaching more and more closely to each other in early Tertiary times, in spite of the imperfectness of the geological record. We at last trace these diverging lines to a few very primitive, generalised, patriarchal groups, which in turn approach each other very closely in structure, and plainly suggest a common Cretaceous ancestor. Whether that common ancestor was an Edentate, an Insectivore, or Creodont, or something more primitive than them all, is disputed. But the divergence of nearly all the lines of our mammal world from those patriarchal types is admirably clear. In the mutual struggle of carnivore and herbivore, in adaptation to a hundred different environments (the water, the land, and the air, the tree, the open plain, the underground, the marsh, etc.) and forms of diet, we find the descendants of these patriarchal animals gradually developing their distinctive characters. Then we find the destructive agencies of living and inorganic nature blotting out type after type, and the living things that spread over the land in the later Tertiary are found to be broadly identical with the living things of to-day. The last great selection, the northern Ice-Age, will give the last touches of modernisation.