I intend to give in this chapter a general outline of the subject which will be discussed in the subsequent ones. This will include a brief history of the great events, in recent geological times, which have modified the evolution of the European fauna by the influence which they have exerted on the course of the successive streams of migration.
The composition of the European fauna is the first item which will have to be taken into consideration. But not only must the existing species of animals be dealt with: the extinct ones, too, at least those which have lived in Europe during late Tertiary times, will be useful for our inquiries. A knowledge of the past faunas is a most important factor in tracing the original home of the European animals.
Where a species first originated, whether this was in one or several places, or, in other words, where it first had its home, cannot be determined with absolute certainty in the present state of our knowledge, but as a rule it can be indicated approximately with a fair amount of precision. In a few instances, species may possibly have had a dual origin. The majority of naturalists doubt that there are any such, but it seems to me that almost the same forces may have acted in different localities on certain forms so as to produce, in very exceptional circumstances, similar species. The vast majority of animals, however, have no doubt originated in one locality; or, we might say, almost all species have but one home.
We may assume that every animal gradually extends its range by migration, as the result of the natural increase of the species necessitating a search for fresh feeding grounds. Every species thus tends to slowly take possession of all the habitable parts of the globe to which it has access. They would all naturally spread from their original homes in every direction, unless prevented by an impassable barrier. We have already learned that to all land animals, the sea acts as such a barrier. Mountains and rivers act also in a similar way, but not to the same extent. It is not difficult to understand also that a forest may be a formidable barrier to a typical inhabitant of the open country and vice versâ, whilst a desert is impassable to almost all terrestrial organisms. Some species are scarcely affected by climate, and flourish equally well in the tropics and in temperate or cold countries; the majority, however, are greatly influenced by it. "No more striking illustration," remarks Merriam (p. 38), "could be desired of the potency of climate compared with the inefficiency of physical barriers, than is presented by the almost total dissimilarity of the North American Tropical and Sonoran Regions, though in direct contact, contrasted with the great similarity of the Boreal Regions of North America and Eurasia, now separated by broad oceans, though formerly united, doubtless, in the region of Behring Sea."
To return to the composition of the European fauna, we now know positively that a number of the mammals and birds inhabiting Central and Eastern Europe are of Siberian origin. How they came, and when, will form the subject for discussion in Chapter V. At present it will suffice to mention that in the superficial deposits belonging to the Pleistocene series of the North European plain have been discovered the remains of many typical members of the Siberian Steppe-fauna. Some of these, such as the Saiga-Antelope (Saiga tartarica), [Fig. 2], still inhabit portions of Eastern Europe, whilst others have retreated to their native land. But it might be asked, how is it known that these species did not originate in Europe, and thence migrate to Siberia? Because if they had originated on our continent, they would have spread there. They would have invaded Northern and Southern Europe, and they would probably have left some remains in Spain, Italy, or Greece. They would also have left some of their relations in Europe; but all their nearest allies, too, are Asiatic. Moreover,—and this completes, I think, the proof of their Siberian origin,—the Pleistocene remains of these animals in Europe become less abundant, and the number of species likewise decreases, as we proceed from east to west. With these remains of Steppe animals are generally associated those of others, which we must also look upon as Siberian emigrants, such as the Pikas or tailless Hares belonging to the genus Lagomys, the pouched Marmots (Spermophilus), and others. Some of them, as I have mentioned, still inhabit Central and Eastern Europe, whilst others have a wider distribution on our continent.
Fig. 2.—The Saiga-Antelope (Saiga tartarica). (From Lydekker's Royal Natural History, vol. ii. p. 298.)
This migration must have been an unusually large one. It has been suggested that the Glacial period had some connection with it, and there can be little doubt, as we shall see later on, that a change of climate probably brought about this great Siberian invasion of Europe. But other causes might tend in the same direction, such as want of sufficient food after a few years of great increase of any particular species. It is not known to what we owe the periodic visits of the Central Asiatic Sandgrouse (Syrrhaptes paradoxus), [Fig. 3], but certain it is that immense flocks of these birds invade Europe from time to time at the present day, just as those mammals may have done in past ages.