Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/294

276 DISTRIBUTION [ANIMAL. first time from the north. This will explain many of the peculiarities of the Palaearctic, Oriental, and Ethiopian regions, and of their several sub-regions, and especially the persistence of low types in those districts which were wholly or partially protected from the competition of more highly organized animals. The Tertiary fauna of North America compared with that of Europe exhibits proofs of a former communication between the two northern continents both in the North Atlantic and North Pacific, but always, probably, in rather high latitudes. This is indicated both by the groups which appear to have originated in one continent and then to have passed across to the other, and also by the entire absence from America of many important groups which abounded in Europe (and vice versa), indicating that the communication between the two hemispheres was always imperfect and of limited duration. The past zoological history of North and South America exhibits a somewhat analogous series of phenomena. Their productions were generally very dissimilar. North America, in closer connection with the great northern continent, made an almost equal advance in the development of the more highly-organized animals ; while South America, for the most part isolated and thus prevented from receiving a constant supply of immigrants from the larger land-areas, developed a series of lowly-organized creatures, the smaller forms of which still constitute its chief zoological feature. The knowledge we possess of Tertiary and post-Tertiary Mammalia thus gives us an important clue to the successive migrations of the various groups of animals from one region to another, and to the geographical changes which rendered such migrations possible. The general result arrived at is, that the great northern continents represent the original seat of mammalian life, and the region of its highest development ; while the southern continents Australia, South America, and Africa have been isolated for varying periods, and, after receiving an immigration of lowly forms, have developed and preserved these to a greater or less ex tent, according as they were more or less completely pro tected from the irruption and competition of higher types. Australia, during the Secondary period, received from the northern continent a stock of Marsupials and perhaps some still lower forms, and, having been since completely isolated, has developed these groups alone into its existing fauna. South America, at a somewhat later period, obtained the ancestors of its Edentata and Rodents ; and though at various times some higher forms entered it from the north, ihese never seem to have been sufficiently numerous to overcome its indigenous fauna. In Africa the case was different. For a long time its Mammalia were probably analogous to those of South America ; but when the great irruption of higher animals took place in the latter part of the Tertiary period, most of these were destroyed, and a few only remain such as the Orycteropus, the Lemurs, and the peculiar Rodents as indications of the character of the primeval fauna. In the peninsula of India a very similar course of events occurred, and the fauna of both these countries now consists mainly of comparatively recent immigrants. (For a fuller discussion of this subject see Wallace s Geographical Distribution of Animals, chapters vi. to xv.) The Birth-place and Migrations of some Mammalian Families and Genera. From the knowledge we now possess of the extinct fauna of most of the great continents, it is possible to determine approximately the original birth place of some now widely distributed groups. The true bears, for example, date back in Europe to the older Pliocene, while in North America they occur only in post- Pliocone deposits. We may conclude, therefore, that they originated in the Old World and are comparatively recent immigrants in America. True horses of the genus Equus are also of older Pliocene date in Europe and of the post- Pliocene, or perhaps newer Pliocene, in America, and are therefore also recent immigrants into the latter country. But it is a curious fact that the most perfect series of ancestral forms of horses occur in the Miocene and Eocene deposits of North America ; whence it would seem probable that the earlier stages of the development of this wonder fully specialized animal were effected in America, whence they passed to the eastern hemisphere, and there attained to the full development of the equine type, again, perhaps, to be transferred to America, to be largely developed there (for remains of eight or ten distinct species have been discovered), and finally to become wholly extinct, while continuing to exist in the Old World, whence the most perfect form has been again introduced, and seems quite capable of maintaining itself in a wild state. Tapirs, though now more abundant in America than in Asia, are an Old World group, going back to the Lower Miocene in Europe, but only appearing in America in the post-Pliocene epoch. The peccaries (Dicotyles), now almost wholly Neotropical, are really a North American group, and probably only entered South America in later Pliocene times. Camels, though now confined to Asia and South America, are really a. North American form, having been largely developed during the Miocene period, whence the true camels appear to have passed into Asia and the llamas into South America. True deer are European from Miocene times, but only appear in America in the later Pliocene and post-Pliocene epochs. Elephants are an Old World type, abounding from the Miocene period in Europe and Asia, but only appearing in America in the later Pliocene and post-Pliocene times. It is possible^ however, that the Eocene Dinocerata of North America may be ancestral forms of Proboscidea, and that, as in the case of the horses, the development of elephants may have begun in America to be subsequently perfected in the larger area of the eastern hemisphere. As a last and curious example we may refer to the marsupial opossums, now exclusively American, but which are certainly recent immigrants from Europe or Asia. No trace of them occurs in American deposits before the post-Pliocene period, while they existed in Europe both in Eocene and Miocene times. The cases now adduced are sufficient to show how much interest attaches to the distribution of the ancestral forms of our existing animals ; but we wait for fuller knowledge of the Tertiary deposits of Asia, Africa, and South America in order to complete the history of these migra tions, and to gain some knowledge as to many other groups whose origin is now involved in obscurity (Geoy. Dist. of Animals, vol. i. p. 153.) DISTRIBUTION OF MARINE ANIMALS. The zoological regions which serve to represent the main facts of the distribution of land animals are evi dently inapplicable to those inhabiting the ocean, except in a few cases where the group is confined to shallow waters or to estuaries, It is true that, as the great continents are separated by the oceans, so the oceans are to some extent separated by the continents, but owing to the superior area of water the separation is far less complete and effective. In the southern hemisphere the Pacific, Atlantic, and Indian oceans freely communicate, and for truly oceanic animals there would seem to be hardly any obstacle against universal distribution. Yet even in this case physical conditions, especially depth and tempera ture, are found to be effective barriers. The fact that the deep waters even of the tropical seas are cold, renders it indeed possible for some temperate or Arctic forms to cross the equator if they can travel at great depths ; but for sur-