SECTION VII.-THE PAST HISTORY OF THE PALEARCTIC MAMMAL-FAUNA Although the palæontological history of Europe, so far as it has been worked out, has been very thoroughly investigated, our knowledge of its extinct mammals, at any rate, is not to be compared with that which has been acquired in the Nearctic Region. This is probably due, to a great extent, to the comparative rarity on this side of the Atlantic of fresh-water lake deposits, the examination of which, in North America, has produced such astonishing results. Passing over the Mesozoic Mammals, which throw very little light on any of the problems involved in the present case, we find in the earliest Eocene beds scanty remains of a fauna containing hardly any members of the existing orders of Mammals. In their place is a series of forms closely resembling one another in possessing five-toed and N plantigrade extremities, furnished with neither hoofs nor claws, but with structures somewhat intermediate between the two. Nevertheless, even among these primitive mammals, it is possible to recognize the germs of the marked characters which at the present day separate the various Orders. In North America, in beds of the corresponding age, a much more ample stock of remains of a similar fauna is met with. Later on, in the Upper Eocene beds a much larger number of Mammals appears, this fauna containing at least a hundred genera, most of them of large size, whereas to-day the European Mammal-fauna consists only of fifty-four genera, and of these more than half are of small size. At this epoch slight distinctions between the European and American forms begin to appear, showing that even at these early times there was a commencing separation between the two great continents. In the earlier part of the Miocene age, so far as we know, no very great changes take place, but at the end of Miocene time we find in several localities wonderful assemblages of fossil Mammals in great abundance and in an excellent state of preservation, which enable us to make a better comparison. Such localities have been discovered at Pikermi in Greece, in the island of Samos in the Ægean Sea, at Maragha in Persia, and, perhaps the most important of all of them, in the Sivalik Hills at the southern base of the Himalayas. This fauna bears a close resemblance to that of the Ethiopian Region in the present state, especially as regards the presence of Giraffes, Gazelles, and other Ungulates. North of the Alps this fauna, although represented, is not nearly so rich, many of the Antelopes and Giraffes being absent and being replaced by various forms of Deer (Cervida), which now commence to be very much more prominent. In the true Sivalik fauna of India there are a good many types which have never yet been found in Europe; such, for instance, as the Camels, which are specially characteristic of the American Tertiary strata. Furthermore, there are found, in the American formations of this age, a large number of forms, such as Bos, Equus, Hippopotamus, and Ursus, which do not appear at all in Europe until the later Pliocene times. When the Pliocene times arrive, we begin to find a preponderating number of still existing genera present in the fossil beds, although the greater number of them have, at the present epoch, retreated southwards into the Oriental and Ethiopian Regions. This southward migration seems to have gone on throughout the Pliocene period, and was probably occasioned by the increasing cold caused by the gradual advent of the great Ice-age, which now began to make itself felt over the whole of the northern part of the globe. Finally, during the Glacial period the fauna assumed nearly its present form, containing large numbers of species that still survive. At this epoch, too, a connection appears to have been formed between the Old and New Worlds in the neighbourhood of Behring Strait, by means of which an interchange of animals took place, and resulted in occasioning the similarity which forms so marked a feature on a comparison of the Nearctic and Palearctic faunas. It is this similarity that has caused certain writers on geographical distribution to unite the Palearctic and Nearctic Regions into one, whereas, as a matter of fact, palæontological evidence seems to show that, out of all the |