Progressive Brain-Development.
The three illustrations now given sufficiently prove that, whenever the geological record approaches to completeness, we have evidence of the progressive change of species in definite directions, and from less developed to more developed types—exactly such a change as we may expect to find if the evolution theory be the true one. Many other illustrations of a similar change could be given, but the animal groups in which they occur being less familiar, the details would be less interesting, and perhaps hardly intelligible. There is, however, one very remarkable proof of development that must be briefly noticed—that afforded by the steady increase in the size of the brain. This may be best stated in the words of Professor Marsh:—
"The real progress of mammalian life in America, from the beginning of the Tertiary to the present, is well illustrated by the brain-growth, in which we have the key to many other changes. The earliest known Tertiary mammals all had very small brains, and in some forms this organ was proportionally less than in certain reptiles. There was a gradual increase in the size of the brain during this period, and it is interesting to find that this growth was mainly confined to the cerebral hemispheres, or higher portion of the brain. In most groups of mammals the brain has gradually become more convoluted, and thus increased in quality as well as quantity. In some also the cerebellum and olfactory lobes, the lower parts of the brain, have even diminished in size. In the long struggle for existence during Tertiary time the big brains won, then as now; and the increasing power thus gained rendered useless many structures inherited from primitive ancestors, but no longer adapted to new conditions."
This remarkable proof of development in the organ of the mental faculties, forms a fitting climax to the evidence already adduced of the progressive evolution of the general structure of the body, as illustrated by the bony skeleton. We now pass on to another class of facts equally suggestive of evolution.
The Local Relations of Fossil and Living Animals.
If all existing animals have been produced from ancestral forms—mostly extinct—under the law of variation and natural selection, we may expect to find in most cases a close relation between the living forms of each country and those which inhabited it in the immediately preceding epoch. But if species have originated in some quite different way, either by any kind of special creation, or by sudden advances of organisation in the offspring of preceding types, such close relationship would not be found; and facts of this kind become, therefore, to some extent a test of evolution under natural selection or some other law of gradual change. Of course the relationship will not appear when extensive migration has occurred, by which the inhabitants of one region have been able to take possession of another region, and destroy or drive out its original inhabitants, as has sometimes happened. But such cases are comparatively rare, except where great changes of climate are known to have occurred; and we usually do find a remarkable continuity between the existing fauna and flora of a country and those of the immediately preceding age. A few of the more remarkable of these cases will now be briefly noticed.
The mammalian fauna of Australia consists, as is well known, wholly of the lowest forms—the Marsupials and Monotremata—except only a few species of mice. This is accounted for by the complete isolation of the country from the Asiatic continent during the whole period of the development of the higher animals. At some earlier epoch the ancestral marsupials, which abounded both in Europe and North America in the middle of the Secondary period, entered the country, and have since remained there, free from the competition of higher forms, and have undergone a special development in accordance with the peculiar conditions of a limited area. While in the large continents higher forms of mammalia have been developed, which have almost or wholly exterminated the less perfect marsupials, in Australia these latter have become modified into such varied forms as the leaping kangaroos, the burrowing wombats, the arboreal phalangers, the insectivorous bandicoots, and the carnivorous Dasyuridae or native cats, culminating in the Thylacinus or "tiger-wolf" of Tasmania—animals as unlike each other as our sheep, rabbits, squirrels, and dogs, but all retaining the characteristic features of the marsupial type.
Now in the caves and late Tertiary or Post-Tertiary deposits of Australia the remains of many extinct mammalia have been found, but all are marsupials. There are many kangaroos, some larger than any living species, and others more allied to the tree-kangaroos of New Guinea; a large wombat as large as a tapir; the Diprotodon, a thick-limbed kangaroo the size of a rhinoceros or small elephant; and a quite different animal, the Nototherium, nearly as large. The carnivorous Thylacinus of Tasmania is also found fossil; and a huge phalanger, Thylacoleo, the size of a lion, believed by Professor Owen and by Professor Oscar Schmidt to have been equally carnivorous and destructive.[189] Besides these, there are many other species more resembling the living forms both in size and structure, of which they may be, in some cases, the direct ancestors. Two species of extinct Echidna, belonging to the very low Monotremata, have also been found in New South Wales.
Next to Australia, South America possesses the most remarkable assemblage of peculiar mammals, in its numerous Edentata—the sloths, ant-eaters, and armadillos; its rodents, such as the cavies and chinchillas; its marsupial opossums, and its quadrumana of the family Cebidae. Remains of extinct species of all these have been found in the caves of Brazil, of Post-Pliocene age; while in the earlier Pliocene deposits of the pampas many distinct genera of these groups have been found, some of gigantic size and extraordinary form. There are armadillos of many types, some being as large as elephants; gigantic sloths of the genera Megatherium, Megalonyx, Mylodon, Lestodon, and many others; rodents belonging to the American families Cavidae and Chinchillidae; and ungulates allied to the llama; besides many other extinct forms of intermediate types or of uncertain affinities.[190] The extinct Moas of New Zealand—huge wingless birds allied to the living Apteryx—illustrate the same general law.
The examples now quoted, besides illustrating and enforcing the general fact of evolution, throw some light on the usual character of the modification and progression of animal forms. In the cases where the geological record is tolerably complete, we find a continuous development of some kind—either in complexity of ornamentation, as in the fossil Paludinas of the Hungarian lake-basins; in size and in the specialisation of the feet and teeth, as in the American fossil horses; or in the increased development of the branching horns, as in the true deer. In each of these cases specialisation and adaptation to the conditions of the environment appear to have reached their limits, and any change of these conditions, especially if it be at all rapid or accompanied by the competition of less developed but more adaptable forms, is liable to cause the extinction of the most highly developed groups. Such we know was the case with the horse tribe in America, which totally disappeared in that continent at an epoch so recent that we cannot be sure that the disappearance was not witnessed, perhaps caused, by man; while even in the Eastern hemisphere it is the smaller species—the asses and the zebras—that have persisted, while the larger and more highly developed true horses have almost, if not quite, disappeared in a state of nature. So we find, both in Australia and South America, that in a quite recent period many of the largest and most specialised forms have become extinct, while only the smaller types have survived to our day; and a similar fact is to be observed in many of the earlier geological epochs, a group progressing and reaching a maximum of size or complexity and then dying out, or leaving at most but few and pigmy representatives.