The view still current among many zoologists and botanists, that animals and plants were driven down into the plain from the mountains of Europe during the height of the Glacial period and there lived together till the return of a more genial temperature, when they retreated to their mountain homes, is a very plausible one. During their sojourn in the plain, the plants and animals--say from Scandinavia--intermingled with those from the Alps; and when the time of separation came, many Alpine forms retired northward with the Scandinavians, while many Scandinavians would go with the Alpines to their home. In this way the similarity between the Alpine and Scandinavian faunas and floras is assumed to have been brought about. These theories, first promulgated by Edward Forbes, were hailed with general satisfaction by the scientific world. Even Darwin says of them (p. 331), that grounded as they are on the perfectly well-ascertained occurrence of a former Glacial period, they seemed to him to explain in a satisfactory manner the present distribution of the Alpine and Arctic productions of Europe. To the present day this view meets with much favour among naturalists. It is somewhat similar to one which has recently been strongly supported by Professor Nehring and accepted by Professor Th. Studer and many others. They have never made it quite clear whether the pre-glacial fauna and flora are supposed to have been absolutely destroyed by the glacial climate, or whether part of them have been able to take refuge somewhere in the south; but the great mass of our Alpine plants and animals are believed to have been derived from the Siberian invasion, which I have fully described in the fifth chapter. This invasion spread over the European plain, and when the climate ameliorated, both animals and plants migrated north and south to the mountains. This view agrees with the earlier theory, except that the adaptation to Alpine conditions would, according to the former, have taken place since the close of the Glacial period, during which time no such modification or change of species seems to have been produced in other parts of the world. The characteristic fauna of the Alps, as has been gathered from the preceding pages, is mainly of Central Asiatic rather than of Siberian origin. Migration to the Alps took place by the Oriental route long before the Siberian invasion. Some of the species of the latter have penetrated to the Alps, but these Siberian species have not given to the fauna of the highest European mountain range the striking character with which we all associate it.