[Illustration: Fig. 16.--Map of Europe. The dotted portions represent, approximately, the course of migration of the Siberian mammals. The principal mountain ranges are roughly indicated in black.]
In the migrations of to-day we perceive the same tendency as in the older ones of which we have fossil evidence, viz., generally a spreading of species on a large scale over new territory, and then a gradual shrinkage towards their original home, with an occasional survival of small colonies in the invaded part. It must not be supposed that this observation applies alone to the Siberian migration. In the case of the Arctic one, precisely the same thing has happened, and we shall see that the Southern (migration from the south) agrees in this respect with the others.
As for the immediate cause of these migrations, it is to be looked for either in the scarcity of food dependent upon a temporary or permanent change of climate, or in an excessive increase in numbers of a particular species. I do not propose to trace back migrations beyond the Pliocene Epoch, or indeed much beyond the beginning of the Glacial period, which is regarded as a phase of the most recent geological epoch, viz., the Pleistocene. During the period in question, we have indirect evidence of one vast migration from Siberia into Europe across the lowlands lying to the north of the Caspian and to the south of the Ural Mountains. There is a general consensus of opinion that this migration took place in Pleistocene times. Professor Nehring thinks that there can be no doubt (p. 222) that the Siberian migrants arrived in Northern Germany after the first stage or division of the Glacial period, and lived there probably during the inter-glacial phase which occurred between the first and second stages--if indeed we look upon this period as being divisible into two distinct stages.
Judging from the evidence of distribution of mammals in pleistocene Europe, Professor Boyd Dawkins came to the conclusion (p. 113) that the climate of our continent "was severe in the north and warm in the south, while in the middle zone, comprising France, Germany, and the greater part of Britain, the winters were cold and the summers warm, as in Middle Asia and North America." "In the summer time the southern species would pass northwards, and in the winter time the northern would swing southwards, and thus occupy at different times of the year the same tract of ground, as is now the case with the Elks and Reindeer." Very different are the views of Professor Nehring on this subject. According to him, the climate in Germany must have been extremely cold and damp, resembling that of Greenland, though perhaps not quite so arctic. Professor Nehring does not at all believe that southern and northern species of mammals could have lived in Central or Northern Europe at the same time; though of this we have undoubted geological evidence (pp. 72-75). He thinks that the supposed commingling of southern and northern types, which has actually been shown by Professor Dawkins to occur, is either due to careless observation or to the fact that some of the species need not necessarily have lived where their bones were found (p. 133).