The various groups of Vertebrates are not dependent on each other in their migrations. Mammals and Birds extend their range with so much greater facility than Reptiles and Amphibians, that the surplus population of our neighbouring continents readily poured into Europe when--owing to changes of climate perhaps--they forsook their original homes.
We observe much the same differences of origin in the various groups of European Invertebrates. The Central European Molluscan fauna, remarks Dr. Kobelt, had already developed from the pliocene--in almost all its details, as regards formation of species and distribution--when the Ice-Age commenced (_b_, i. p. 162). Certain very interesting dislocations, however, in the range of land mollusca can be proved to have taken place about that time. Thus, as Dr. Kobelt has pointed out, the genus _Zonites_, which is now almost confined to the south-east of Europe, occurs in inter-glacial deposits in the valley of the Neckar, and even as far west as the Seine. If we might judge from this single instance, a molluscan migration from the east to the west seems to have occurred either in early or pre-glacial times. That _Helix pomatia_ has migrated only comparatively recently from the East to Western Europe is rendered probable by its general range in northern and western Europe, but I cannot agree with Dr. Kobelt in the belief that _Helix aspersa_ is of an equally recent origin in the North. No matter whether it has been found fossil or no, its range in the British Islands points to its having penetrated to Ireland when the latter was still connected with the Continent by way of England. Its migration from the Mediterranean dates therefore from early pleistocene or late pliocene times.
In referring to the sixty-five species of Land and Freshwater Mollusca which have been described from the continental "Loess," Dr. Kobelt states (p. 166) that this fauna has certainly not a steppe-character. It does not therefore strengthen Professor Nehring's view that Europe during the deposition of the loess had a climate comparable to that of the Siberian steppes. The Glacial period had hardly any effect on the molluscan fauna of Europe. Dr. Kobelt believes in a certain movement of that fauna from the least favourable areas, with a subsequent re-immigration; but even that could not have taken place on a large scale. Nothing like a destruction of the fauna occurred, as far as we know from fossil evidence.