1916] Taylor: Beavers of Western North America 415 



leads to the discovery of new problems, the evolution of new theories, 

 and the coordinated accumulation of new facts. Ill effects, how- 

 ever, may sometimes be realized as a result of it. A field of investiga- 

 tion the resources of which are by no means exhausted may be for- 

 saken by those best fitted to prosecute researches therein on the 

 ground that some other field looks more promising. 



The studies of chorology, that is to say, of the geographical dis- 

 tribution of living forms, and of the relations of the living organism 

 to its natural environment, hold, in the opinion of the writer, posi- 

 tions in scientific interest subordinate to those to which their abundant 

 and practically unexplored resources would seem appropriately to 

 assign them. 



With the swing of the pendulum of scientific interest away from 

 these fields, work on what is probably an important condition in 

 polytypic evolution, namely, isolation, has practically ceased. The 

 large place in organic evolution which may be filled by this condition 

 has been emphasized by Wagner (1868), Dixon (1885), Romanes 

 (1886), Gulick (1905), and Jordan (1905). 



Questions arise immediately: What is isolation? Has it any 

 importance in organic evolution? Is it not all-important? Have 

 not the mutation and Mendelian concepts of the last few years done 

 away with the necessity for postulating it at all as a condition of 

 evolution? If isolation is the fictitious result of speculative induc- 

 tion, the sooner the concept is thrown overboard the better. If, on 

 the other hand, it is of importance as a factor in the evolution of any 

 group of living forms, it ought to receive broader recognition than 

 it has heretofore. 



It is the intention to present here some of the facts which seem 

 to demand consideration, and which are drawn, not only from the 

 study of beavers, but also from the geographical distribution and 

 relationships of certain other families of west American (chiefly 

 Calif ornian) mammals. It is the hope of the writer that he may be 

 able to emphasize : The importance of the study of isolation and 

 certain related problems; the pertinence and indispensability of 

 evidence from zoogeography. 



DETAILS OF TREATMENT 



In the preparation of this paper the inadequacy of material has 

 been sharply felt. Still, it has been possible to get together a greater 



