renders it impossible that identical conditions could have been acquired twice independently. The fact that the two groups of camels agree in a large number of points, in which they differ from all other animals of the same class, must be taken as a proof of near blood-relationship. Near blood-relationship means common origini.e., one of the two groups of camels must be descended from the other, or both groups must be descended from some common ancestors, which were already camels. In other words, either the newworld camels must be descended from the old-world camels, or vice versa; or both must be descended from camels that formerly lived elsewhere, but are now extinct. Our problem is now becoming more clearly defined, and we have to consider the means of migration of mammals. The only means of migration is by walking; for although most of them can swim, it is only for short distances, and it is doubtful whether any land mammal can swim across an arm of sea fifty miles wide. Captain Webb's swim across the Channel has perhaps never been beaten by a land mammal. The practical proof of the efficacy of the sea as a barrier to migration is seen in the fact of the absence in most oceanic islands of indigenous mammals, except bats. To put it in plain words, if mammals are to get from one place to another, they must walk. The only explanation possible is through fossils, which thus have a new interest, depending on what parts of the earth we find them in. The evidence of fossils with regard to camels is very imperfect, but still points in a fairly definite direction. Fossil camels are found in South America, in Brazil; in North America, in Texas, California, Kansas, and Virginia. In Asia, in the Himalayas, Merycotherium, a large fossil camel, is found widely distributed over Siberia, extending to the extreme coast. Now, there was almost certainly a former land connection between Asia and North America, across the Behring Straits, which are narrow and shallow. Hence the conclusion is, that there is strong reason for holding that camels originated in North America, and thence spread in two directions, southwards to South America, and westwards through Asia; and that their areas of distribution, though now disconnected, were once continuous. B. MARSUPIALS.-These constitute a large group of animals, of which there is a great variety of forms, the kangaroo and opossum being perhaps the best known examples. Marsupials are a well-marked group, which in their habits, appearance, and structure, especially as regards the skeleton and teeth, curiously simulate the higher divisions of mammals. Carnivorous, insectivorous, and herbivorous forms are all well established and differentiated. The ant-eater or Myrmecobius, and fruit-eater or Phalanger, are found in this group, which is characterised by low organisation and great tenacity of life. Marsupials occur in two chief regions : (i.) Australian region. The wombats and Myrmecobius occur only in Australia and Tasmania, kangaroos and phalangers extending northward to New Guinea and adjacent islands; phalangers to Timor, the Moluccas, and Celebes. (ii.) American region. -The opossums are most numerous in the forest region of Brazil, south of the river La Plata; also west of the Andes in Chili. Their distribution extends northwards to Mexico, Texas, and California; and in the States from Florida to the Hudson river, and westwards to the Missouri. Marsupials form a good example of discontinuous distribution, the explanation of which is yielded by fossils. Opossums are found in the Tertiary deposits of England, France, in other parts of Europe, and in North America. The Cretaceous period shows no trace of them, but in the Jurassic, and in the yet older Triassic, at the base of the Secondary series, large numbers of mammalian remains of small size have been found, which are considered to represent the early phase in marsupial development. Here the starting-point or birth place appears to have been in the Old World, and the group to have migrated southwards. C. TAPERIDÆ, or tapirs, are found in the equatorial forests of South America, in the Andes of Ecuador, in Panama and Guatemala, and also in the Malay Peninsula, Sumatra, and Borneo. Geological evidence shows that during the Miocene and Pliocene times, tapirs abounded over the whole of Europe and Asia, and their remains are found in the Tertiary deposits of France, India, Burmah, and China. both North and South America fossil remains of In tapirs occur only in caves and deposits of the Post Pliocene age, showing that they are comparatively recent immigrants into that continent, perhaps by means of the Behring Straits again. The climate even now is much milder than on the north-east of America, and perhaps was warm enough in late Pliocene times to allow emigration of tapirs, which were driven south to the swampy forests of the Malay region. CONCLUSION.- On the whole, then, the evidence afforded us by fossils is not so complete as we should wish, and we have seen that from the necessity of the case this must be so. However, evidence is steadily accumulating, and such evidence as we have is not merely favourable, but in some instances remarkably complete. Indeed, since the date of publication of the "Origin of Species," in 1859, our knowledge has increased, and evidence has accumulated so markedly, that it has been said by a highly competent authority, that if the doctrine of Evolution had not existed, palæontologists would have been compelled to invent it as the only possible explanation of the facts determined. Again, we are not aware of the existence of palæontological facts which can be demonstrated to be inconsistent with the theory; while the explanation which they afford of new and previously unstudied problems, such as some of the questions of geographical distribution we have touched upon, is evidence of a strong nature in support of the theory. Professor Huxley, with regard to this subject, says: "The primary and direct evidence in favour of Evolution can be furnished only by palæontology. The geological record, so soon as it approaches completeness, must, when properly questioned, yield either an affirmative or a negative answer: if Evolution has taken place, there will its mark be left ; if it has not taken place, there will lie its refutation." |