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:--
1. Pupa muscorum. 2. Chondrula tridens. 3. Cionella lubrica. 4. Patula ruderata. 5. Do. rotundata. 6. Helix striata. 7. Do. hispidia. 8. Do. tenuilabris. 9. Helix pulchella. 10. Do. hortensis. 11. Do. obvoluta. 12. Hyalinia radiatula. 13. Succinea oblonga. 14. Limnæa peregra. 15. Clausilia sp. 16. 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.