But it seems to me that there is no necessity for assuming such drastic changes of climate to have taken place at all (compare pp. [75]-[80]). We really have no idea under what precise climatic conditions the Siberian mammals lived in their original home. The only thing we can be certain of is that the smaller burrowing mammals would not have chosen a wood to live in, if they could possibly help it. Prairies, or sand-dunes with short grass or shrubs, such as abound in Europe near the sea-coast, would suit these species perfectly. If we suppose Northern Germany to have been covered by sea (p. [156]) during part of the Pleistocene Epoch, forests would probably not have grown there for a very considerable time afterwards, owing to the excessive salinity of the soil, but a tract of sandy country would have been left on the retreat of the sea. Possibly a slight change of climate in the original home of these steppe-species may have reduced their habitable area, and thus caused their migration into Europe.
But this migration problem cannot be solved without tracing the mammals to their place of origin and investigating their early history. This I shall attempt to do presently; meanwhile, it would be interesting to note whether other groups of animals support Professor Nehring's steppe-theory.
Among groups other than mammals, the most important, for the purpose of drawing conclusions as to former physical conditions and climate, are the mollusca. Their remains have been well preserved, and are easily identified. Though Professor Nehring argues that the molluscs found along with the small mammals harmonise perfectly with the assumption of a steppe-climate (p. 212), I cannot at all agree with him. He enumerates the following sixteen species as having been discovered by him:—
- Pupa muscorum.
- Chondrula tridens.
- Cionella lubrica.
- Patula ruderata.
- Do.rotundata.
- Helix striata.
- Do.hispidia.
- Do.tenuilabris.
- Helix pulchella.
- Do.hortensis.
- Do.obvoluta.
- Hyalinia radiatula.
- Succinea oblonga.
- Limnæa peregra.
- Clausilia sp.
- Pisidium pusillum.
Only two of these can be looked upon as typically northern species, viz., Patula ruderata and Helix tenuilabris, though both of them are still found living locally in Germany. Some of the others are decidedly southern species, like Chondrula tridens, Helix obvoluta, H. rotundata, and H. striata. All the rest live and flourish, for example, in Ireland at the present day, where, as we all know, anything but a dry steppe-climate prevails.
Dr. Kobelt quite agrees with me in thinking that the remains of the mollusca found along with the so-called "steppe-mammals" afford no proof of a steppe-character of the country at the time when they were alive (p. 166). Nor do the mollusca which have been found in England in the Forest-Bed and the succeeding pleistocene strata support such a view. The Forest-Bed, generally regarded as belonging to the Upper Pliocene, I believe to be an inter-glacial pleistocene deposit—contemporaneous with the loess formation in Germany. Of fifty-nine species of land and freshwater mollusca which have been discovered in this bed, forty-eight species, according to Mr. Clement Reid (p. 186), are at present living in Norfolk, six are extinct, two are continental forms living in the same latitudes as Norfolk, and the other three are all southern forms. Not a single species has a particularly northern range. Of the land and freshwater mollusca of the South of England in the succeeding pleistocene deposits, six species are now no longer living in the British Islands, but only one (Helix ruderata) can be looked upon as an Arctic or Alpine form. After this short digression on the mollusca, I will briefly recapitulate what is known about the early history of the Siberian mammals, which will assist us in tracing the cause of their migration to Europe.
We have in Siberia problems quite as difficult of solution as the European ones. Volumes have been written to explain the former presence of Arctic mammals like the Reindeer in Southern Europe, and the most extraordinary demands on the credulity of the public have been made by some geologists in their attempts to account for this comparatively simple problem. In Northern Asia a somewhat similar phenomenon, but much more difficult of explanation, has taken place. Mammals have been found fossil in recent geological deposits in localities where they do not now occur, and apparently the Siberian and the European deposits are of about the same age. Now, however, comes the extraordinary difference. In Europe the Arctic mammals went southward, but in Siberia the Southern ones went northward. Not only do we find the Saiga-Antelope, Tiger, Wild Horse, European Bison, Mammoth, and Rhinoceros in the extreme north of the mainland of Siberia; their remains have even been obtained in the New Siberian Islands. As these islands are situated in the same latitude as the northern part of Novaya Zemlya,—indeed, not far south of the latitude of Spitsbergen,—the fact of such huge mammals having been able to find subsistence there at apparently quite a recent geological period seems an astounding fact. It may be urged that their bones might have been carried so far north by ice, or by some other equally powerful agency. But Tcherski and all other palæontologists who have examined these northern deposits are unanimous in the belief that these herbivores and carnivores lived and died where their remains are now found. "It is evident," says Tcherski (p. 451), "that these large animals could only have lived in those extremely northern latitudes under correspondingly favourable conditions of the vegetation, viz., during the existence of forests, meadows, and steppes." He also is of opinion that the moist climate which evidently prevailed in Europe during Post-tertiary (Pleistocene) times must have modified the Siberian climate in so far as to render it milder. The existence of the Aralo-Caspian basin ([Fig. 12], p. [156]) must also have tended in the same direction. It appears then that, at the time when plants and animals are believed to have retired southward in Europe before the supposed advancing Scandinavian ice-sheet, no agency existed in North Siberia which was able to suppress and to annihilate the forest and meadow vegetation, and drive away the fauna connected with it. We know, continues Tcherski, that such genera as Bison, Colus (Saiga), Rhinoceros, Elephas, and Equus are met with in all horizons of the diluvium of West Siberia. He therefore comes to the conclusion (p. 474), that these and other facts imply that the retreat of the North Asiatic fauna commenced about the end of the Tertiary Era (Pliocene), and that it was continued very slowly throughout the Post-tertiary (Pleistocene) Epoch, without any visible changes in its southward direction, even during the time of the most important glacial developments in Northern Europe. Only after the conditions disappeared which had produced the augmentation of an atmospheric moisture, did the climate of North Siberia become deadly to a temperate fauna and flora. Tundras then spread over the meadow-lands and remnants of forests, whilst arctic animals replaced the large ungulates and carnivores which had wandered far away from their native southern home.
This is Tcherski's explanation of the extraordinary events which he has chronicled, after years of the most arduous labour and under conditions of peculiar hardship. And though his work cannot be over-estimated, and his opinions should receive the most careful consideration, yet I fear the explanation will not be looked upon as entirely satisfactory. Every one will agree with him that the climate of Siberia must have been greatly moister in pliocene and pleistocene times than it is now. The Aralo-Caspian covered a vast area of South-western Siberia. Freshwater basins existed along the east of the Ural Mountains, while Central Asia was studded over with a number of large lakes, which have now almost entirely vanished. But that the generally assumed refrigeration of Europe must have had a chilling effect on the Siberian atmosphere seems to me evident. That the whole of Northern Europe should have been made uninhabitable owing to the advance of the Scandinavian ice-sheet, while North Siberia at the same time supported forests, meadows, and a temperate fauna, is incredible. At the approach of winter, at any rate, the animals would have been driven southward for thousands of miles to seek shelter from the snows and cold and to obtain nourishment, and it would scarcely have been possible for them to undertake such vast migrations at every season. Professor James Geikie's suggestion (p. 706), that the Mammoth and Woolly Rhinoceros could have survived the Pleistocene Epoch in Southern Siberia, does not appear to solve the problem, as that part of Asia must have participated in the great cold which is said to have prevailed all over Europe.
Let us now concede, for the sake of argument, that the current views regarding the pleistocene climate of Europe are correct. We are told by Professor Geikie that the climate of Scotland during part of the Pleistocene Epoch was so cold, that the whole country was buried underneath one immense mer de glace, through which peered only the higher mountain-tops (p. 67). If this was the state of climate in close proximity to the Atlantic, it must probably have been still more severe on the European continent. Now at the present time Siberia has the reputation of being the coldest country in the world, and the mercury of the thermometer is said to remain frozen for weeks during winter, even in the south.