^2 HISTORY OF THE EUROPEAN FAUNA. 



phenomenon. But we must not judge the climate 

 of Europe by their presence alone. As I shall 

 explain more fully in Chapter V., these species 

 invaded Europe owing to two circumstances. Firstly, 

 because the climate of Siberia was becoming colder, 

 necessitating a southward movement, with a con- 

 sequent over-population in a reduced area; secondly, 

 because a new short route to Europe had been 

 opened up for them about the same time (see 

 p. 221). An invasion of Europe therefore took place 

 from east to west. Similar invasions occur even at 

 the present day, though not caused by a change in 

 our climate, for every now and then immense flocks 

 of the Siberian Sandgrouse emigrate to our continent. 

 The mammalian migrants referred to are not to be 

 looked upon as constituting the whole of our fauna 

 at that time. Europe had a fauna of its own, and 

 these invaders merely mingled with our animals. 

 There was, no doubt, a keen struggle for existence, 

 as the result of which the weaker in many cases 

 succumbed. The hypothesis, however, that these 

 Siberian migrants occupied an empty continent, 

 forsaken by its pre-glacial inhabitants, is not sup- 

 ported by any facts. 



All those who have investigated the pleistocene 

 fauna have been struck by the extraordinary mixture 

 of northern and southern types of animals. Professor 

 Dawkins attempted to explain these facts by the sup- 

 position (p. 113) that "in the summer time the southern 

 species would pass northwards, and in the winter 



