OF OLD AND NEW WORLD 215 



that the distinction between them, and even the rep- 

 resentation of the same genus on the two conti- 

 nents by clearly different species, is due to the sub- 

 sequent separation of the two regions in glacial and 

 post-glacial times, and the variations which isola- 

 tion, a difference of climatic conditions, and their 

 general environment have brought about in the 

 lapse of time. 



