1916] Taylor: Beavers of Western North America 489 



originally uniform stock as required by Wagner's isolation theory. If 

 any one of the following factors, (1) "advantage of divergence" 

 as Darwin used the term, (2) De Vriesian mutation, (3) Men- 

 delian segregation, (4) physiological selection, has been operative 

 in the differentiation of higher vertebrates, it is not apparent in 

 the data of distribution. In these higher groups differentiation 

 becomes, sooner or later, a part of the connotation of geographic 

 isolation. 



12. A corollary of the proposition just stated is that in beavers, 

 and in higher vertebrates generally, geographic range is exactly as 

 characteristic of species as any physical or psychical attribute. 



13. Any definite conclusions regarding certain problematic cases 

 would be, premature. Much more testimony must be taken before 

 a decision is reached. Such rare occurrences as that of Peromyscus 

 maniculat us onus with its closest relative Peromyscus rnaniculatus 

 austerus in the same territory are susceptible of interpretation 

 according to one of two alternatives: (1) the case may be one of 

 speciation in accordance with Darwin's, De Vries', or Romanes' 

 theories, the two species being preserved intact through Mendelian 

 segregation; (2) the speciation may have proceeded according to 

 Wagner's theory of geographic isolation (that is, by migration, 

 geographical isolation, and differentiation) ; the physiological isola- 

 tion between them may be but partial; and there may have been a 

 re-invasion on the part of one into the range of the other. 



14. Certain experimentalists, as Tower and MacDougal, have 

 subjected parent generations of insects and plants to unaccustomed 

 environmental stimuli, the offspring showing new characters and 

 assumed to represent new specific types. The distribution of the 

 higher vertebrates seems to make it certain that in the majority 

 of instances differentiation follows migration, and, clearly in most 

 cases, subjection to different environmental conditions. It looks 

 as if in both cases heritable variations are produced as a direct 

 response to external stimuli, the response in each case depending 

 on the nature of the organism. 



15. If this be the case, geographic isolation works no less by 

 segregating different environments than by separating different lots 

 of individuals. Stated differently, geographic isolation conditions 

 differentiation through its maintenance of the mutual interrelations 

 of organism and environment through long periods of time. 



16. Zoogeography does not furnish evidence whether or not 



