Let us for a moment reconsider some instances of mammalian migration from Siberia, with a view to studying more closely the nature of these great events. I mentioned the fact that some of the Siberian migrants have remained in England, that more have settled down permanently on our continent, but that many others have either become entirely extinct or do not live any longer in Europe.
Of the mammals which made their appearance in Great Britain in recent geological times, i.e., during and since the deposition of the Forest-Bed for example, the following species probably came direct from Siberia across the plains of Europe, as already mentioned (p. [95]):—
Of the arrival of many of these in Europe we have geological proof, as they have left their bones in recent pleistocene deposits, and are unknown from older European strata. The remote ancestors of others, such as Sorex and Lagomys, no doubt lived in Europe, but the recent species probably had their original homes in Asia. It is evident that in recent geological times there existed no active centre of origin for mammals in Europe, and that our continent was largely dependent on the neighbouring one for the supply of its mammalian fauna. A shifting of the centre of development from Europe to Asia appears to have taken place occasionally, as already mentioned (p. [45]). Mr. Lydekker has drawn attention to the fact that though the remote ancestors of the Elephantidæ resided in Europe, neither the latter continent nor North America was the home of the direct ancestor of any of the true Elephants. Similarly, though we have had our Sorex in Europe from the Upper Eocene and Lagomys from the Middle Miocene, the geographical distribution of Sorex vulgaris and Lagomys pusillus does not support the view that they are of European origin and have migrated to Asia. Their absence from most of the European islands indicates either an extremely recent origin or a recent immigration from Asia, and the latter view seems to me much the more probable.
No less than twenty-six species of the Siberian mammals penetrated as far west as the British Islands, and nine of these still inhabit Great Britain. Some of the remaining seventeen species probably lived only for a very short time in England, and the rest gradually became extinct one by one. This process of extinction of the aliens still continues. The Beaver (Castor fiber) has died out within recent historic times. We possess legends and uncertain historic records pointing to the existence of the Reindeer in Scotland as recently as about seven centuries ago. But much the same state of things has happened on the Continent. The Glutton (Gulo luscus), which still lived in Northern Germany last century, has now entirely vanished from that country, as also the Reindeer. The Lemmings have found an asylum in Scandinavia. The Musk-Ox (Ovibos moschatus) has disappeared not only from Europe but also from Asia, and is now confined to Arctic America and Greenland. The Horse no longer occurs in Europe in the wild state, and the Saiga Antelope (Saiga tartarica) has retreated to the Steppes of Eastern Europe and Western Siberia.
As we proceed more and more eastward across Central Europe, we find that a larger and larger percentage of the Siberian migrants have adopted the new country as their permanent home, though in France and Germany, as well as in Austria, we have evidence that a great number of Siberian species, which formerly lived there, have either become entirely extinct, or have retreated towards the land of their origin. There is a prevalent belief that these migrants have taken refuge on the higher European mountain ranges, but this idea is altogether erroneous, as will be shown in the chapter dealing with the origin of the Alpine fauna.
One of the Jerboas (Alactaga jaculus) occurs fossil as far west as Western Germany, but it is now confined to Russia and Western Siberia. The Bobak marmot (Arctomys bobak), which has a similar range now, probably inhabited France in former times. A Siberian species which has retreated but little is the Hamster (Cricetus vulgaris). Its fossil remains have been found in Central France, but it does not now occur west of the Vosges Mountains.
It appears, therefore, as if a wave of migration had swept over Central Europe from east to west, that those species which were able to adapt themselves to the new surroundings had remained, and as if the rest had died out or were gradually retreating to the east.
Ornithologists are well acquainted with the fact that in some years there is an unusually large exodus from Eastern Europe and Siberia of birds; and that species like the Waxwing (Ampelis garrulus) then appear in great numbers. But the appearance of this bird in Western Europe is not looked upon as so remarkable as that of Pallas's Sandgrouse (Syrrhaptes paradoxus, [Fig. 3], p. [42]), a typical inhabitant and resident of the Arctic Steppes. The last great irruption took place in 1888, and many birds reached even the extreme west of Ireland in May and June of that year. A few weeks before, it had been announced to the German papers that large flocks of this peculiar pigeon-like bird had arrived in the eastern provinces; and though the vast majority vanished as quickly as they had come, a certain number remained for a year or so in the newly visited countries, and some even bred in England.
Twenty-five years before, in 1863, a similar migration had occurred, though not perhaps on quite such a vast scale, and a few small flocks had made their appearance in Western Europe on several occasions between these dates.