PRELIMINARY CONSIDERATIONS. 4! 



cene 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 asso- 

 ciated 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 (Spennophilus), and others. Some of them, 

 as I have mentioned, still inhabit Central and Eastern 

 Europe, whilst others have a wider distribution on our 

 continent. 



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 parti- 

 cular 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. 



The Siberian migrations will be spoken of in the 

 subsequent pages, as the Siberian element of the 

 European fauna. These migrations, however, are not 

 the only ones which reached Europe from Asia. 

 The sixth chapter deals with migrations which have 



