38 HISTORY OF THE EUROPEAN FAUNA. 



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 versa, whilst 

 a desert is impassable to almost all terrestrial organ- 

 isms. 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 



