82 ISOLATION AS A FACTOR 



fishes, representing nearly every genus or sub- 

 genus of the shore-water of Mexico. Each floral 

 region of the northern hemisphere has its char- 

 acteristic form of most of the widespread genera 

 of trees or shrubs. Wherever a distinct barrier 

 exists, geminate species may be found on the 

 two sides of it, unless for one reason or another 

 one of these forms has failed to maintain itself 

 in the struggle for existence. If the barrier is 

 imperfect, the two species are likely to intermin- 

 gle, giving an intergradation of forms. The 

 absence of such connecting series is the only dis- 

 tinction between a species and a subspecies or 

 geographical variety which many naturalists rec- 

 ognize. A subspecies that lives permanently 

 in the same region coincident in range with the 

 species from which it springs is unknown in 

 zoology. 



DARWIN'S VIEW OF THE ROLE OF ISOLATION 



Assuming that this view of the relation of geo- 

 graphical distribution to species-forming is a cor- 

 rect one, it is interesting to note the attitude of 

 Darwin in regard to it. 



It is clear that Darwin had the basal concep- 

 tion of the views here set forth. His own work 

 in South America and that of Wallace in the 

 East Indies yielded similar conclusions, although 

 with Darwin geographical studies were subordi- 

 nated to other forms of evidence of the transfor- 

 mation of species. Isolation Darwin considered 



