1916] Taylor: Beavers of Western North, America 483 



large and heterogeneous, and results that hold good for certain 

 forms of life may not be true of others." The variety of the living 

 world seems to be no less characteristic than its unity; so it would 

 seem appropriate that workers in science should be very charj of 

 launching generalizations as universals. 



Feeling, however, that one of the most pressing of present-day 

 needs in biology is the correlation of the work of the "experi- 

 mentalist" and the "naturalist", the writer is led to submit the 

 following suggestions, incomplete though they may be. 



When we face the question of the manner in which geographical 

 isolation works, some of the results of Tower's experiments with 

 beetles of the genus Leptinotarsa (1906) and of MaeDougal's work 

 with plants of the genus Raimannia (1906, p. 422) are of great 

 interest. It will be remembered that Tower, by subjecting his 

 beetles to varying conditions during the growth-period of the germ- 

 cells, effected the production of new species and new characters. 

 Although the parents were unaffected, their germ-cells were modi- 

 fied, and the offspring grew up different. MacDougal injected salt 

 solution into the ovules of Raimannia just previous to fertilization, 

 securing potentially new species as a result of the chemical and 

 osmotic action exerted on unfertilized ovules. Tower, comment- 

 ing on this case (1906, p. 295), says: "These results of Mac- 

 Dougal's exactly confirm in plants the results that I have obtained 

 in these beetles, so that the point is now doubly certain that herit- 

 able variations are produced as the direct response to external 

 stimuli." 



Our studies of the distribution of higher vertebrates invite the 

 belief that in the majority of cases differentiation follows migration 

 and, clearly in most instances, the exposure to different environ- 

 mental conditions. There is thus suggested a possible partial agree- 

 ment between these two apparently widely sundered classes of 

 observational facts. What has caused the differentiation which we 

 observe in the higher vertebrates? The application of the prin- 

 ciples of Independent Generation, Independent Variation, or Amixia 

 (these three synonymous terms standing for differentiation under 

 similar conditions in virtue of geographic isolation alone) to these 

 classes of organisms is probably limited. These animals usually 

 range so widely that there are obvious environmental differences 

 associated with difference in species. The facts of geographical 

 distribution of higher vertebrates do not militate against Tower's 



