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Show 350 DARWINISM CHAP. surprised that many widespread forms in either continent have not crossed into the other; and that while the skunks (Mephitis), the pouched rats (Saccomyidre), and the turkeys (Melcagris) are confined to Amer·ica, the pigs and the hedgehogs, the true flycatchers and the pheasants are found only in the Euro-Asiatic continent. But, just as there have bem1 periods which facilitated intermigration between America and the Old \Vorld, there have almost certainly been periods, perhnps of long duration even geologically, when these continents have been separated by seas as wide as, or even wider than, those of the present day ; and thus may be explained such curious anomalies as the origination of the camel-tribe in America, and its entrance into Asia in comparatively recent Tertiary times, while the introduction of oxen and bears into America from the Em·o-Asiatic continent appears to have been equally recent.1 vV c shall find on examination that this view of the general permanence of the oceanic and continental areas, with constant minor fluctuations of land and sea over the whole extent of the latter, enables us to understand, and offer a rational explanation of, most of the difficult problems of geographical distribution ; and further, that our power of doing this is in direct proportion to our acquaintance with the distribution of fossil forms of life during the Tertiary period. We must, also, take due note of many other facts of almost equal importance for a due appreciation of the problems presented for solution, the most essential being, the various powers of dispersal possessed by the different groups of animals and plants, the geological antiquity of the species and genera, and the width and depth of the seas which separate the countries they inhabit. A few illustrations will now be given of the way in which these branches of knowledge enable us to deal with the difficulties and anomalies that present themselves. The Distribution of Mars·upials. This singular and lowly organised type of mammals constitutes almo. t the sole representative of the class in Australia 1 For some details of these migrations, see the author's Geographical Distribution of Animals, vol. i. p. 140; also Heilprin's Geographical and Geological Distribution of Animals. XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 351 and New Guinea, while it is entirely unknown in Asia, Africa, or Europe. It reappears in America, where several species of opossums arc found; and it wa. long thought necessary to postulate a direct southern connection of these distant countries, in order to account for this curious fact of distribution. vVhen, however, we look to what is known of the geological history of the marsupials the difficulty vanishc.. In the Upper Eocene dcpo. its of \Vestcrn Europe the remains of several animals clo ely allied to the American opossums have been found ; and as, at this period, a very mild climate prevailed far up into the arctic regions, there is no difficulty in supposing that the ancestors of the group entered America from Europe or N orthcrn Asia during early Tertiary times. Bnt we must go much further back for the origin of the Anstmlian marsupials. All the chief types of the higher mammalia were in existence in the Eocene, if not in the preceding Cretaceous period, and as we find none of these in Australia, that country must have been finally separated from the Asiatic continent during the Secondary or Mesozoic period. Now during that period, in the Upper and the Lower Oolite and in the still older Trias, the jaw-bones of numerous small mnmmalia have been found, forming eight distinct genom, which arc believed to have boon either marsupials or some allied lowly forms. In North America also, in beds of the Jurassic and Triassic formations, the remain of an equally great variety of these small mammalia have been discovered; and from the examination of more than sixty specimens, belonging to at least six distinct genera, Professor Marsh i of opinion that they represent a generalised type, from which the more speciali ed marsupials and insectivora were devc1oped. From the fact that very similar mammals occur both in Europ and America at corresponding periods, and in beds which represent a long succession of geological time, and that dnring the whole of this time no fragments of any higher form . have been discovered, it seems probable that both the northern continents (or the larger portion of their area) were then inhabited by no other mammalia than these, with perhaps other equally low types. It was, probably, not later than the Jurassic age when some of these primitive marsupials were able to enter Australia, where they have since |