Now with regard to the land-ice theory, several serious difficulties present themselves in connection with the origin of the European fauna. In the first place, as the climate renders Northern Siberia almost uninhabitable for mammals at the present day, how much more severe must it have been during the time of the maximum glaciation in Europe. As the then existing fauna was not driven into Europe, where could it possibly have survived? Secondly, how can we reconcile the contemporaneous existence of a great inland sea (the Aralo-Caspian) containing survivals of mild Sarmatic times with an immense glacier almost touching it on its northern shores? How did one of the most characteristic species of that sea, _Dreyssensia polymorpha_, come to make its appearance in the lower boulder-clay of Prussia and then disappear in the upper? And finally, how are we to explain the sudden appearance of a Siberian fauna after the deposition of the lower boulder-clay, except by the removal of a barrier which had prevented their egress from Siberia?
If we assume that the continental boulder-clay of Russia has been formed in the manner so ably explained by Murchison, de Verneuil, and von Keyserling, viz., by a sea with floating icebergs, the temperature of Siberia might have been higher than at present, and have supported a fauna in more northern latitudes.
The contemporaneousness of the deposits of this sea with those of the Aralo-Caspian is also rendered more intelligible. If we suppose, moreover, the connection between the Aralo-Caspian and the White Sea (Fig. 12, p. 156) to have existed at this time, we possess an explanation of the method of migration of the Arctic marine species into the Southern and of the Caspian species (_Dreyssensia_) into the Northern Sea.
An inter-glacial phase is believed to have supervened after the deposition of the lower boulder-clay, and it is during this period that the Siberian species first appeared in Central Europe. If we assume then that the retreat of the Northern Sea (Fig. 13, p. 170) opened up a passage for the Siberian fauna, we have in this very fact also an explanation of the extraordinarily large exodus of Asiatic mammals, because the great reduction of the marine area in Northern Europe would have had an important influence in lowering the temperature in Asia. Only a sudden change of climate in Siberia could have brought about the migration of the vast hordes of Asiatic mammals whose remains we find in Central and Western Europe in deposits of that period.