THE EVOLUTION OF LIVING BEINGS. 153 



This has long been conceded for varieties" ; it was 

 BRIQUET who first claimed its validity for species and he 

 is in my opinion perfectly right. 



This way of a polytopic origin of identical types, ma- 

 kes it possible to divest migration of many very un- 

 certain attempts to explain the present occurrence of 

 identical types in very far distant places, but does not 

 tend to make the present geographical distribution of 

 animals and plants more easy of explanation in detail 

 than it formerly was. 



Geographical distribution is, just like evolution in 

 the past, a historical problem which can never be 

 reconstructed completely, but from which we can de- 

 rive no more than the very general underlying prin- 

 ciples. 



