414 University of California Publications in Zoology [Vol.12 



G. Comparisons of certain American beavers 437 



I. Castor canadensis phaeus Heller, from Admiralty Island, Alaska 437 

 II. Castor canadensis leucodonta Gray, from Vancouver Island, 



British Columbia 440 



III. Castor canadensis pacificus Rhoads, from the Mainland of Brit- 



ish Columbia and Washington 442 



IV. Castor subauratus subauratus Taylor, from the San Joaquin 



Valley, California 446 



V. Castor canadensis frondator Mearns, from the Colorado and 



San Pedro rivers, Mexico 454 



H. Outline of the history of the beavers 457 



I. Summary of relationships of certain North American beavers 460 



I. Some difficulties to precise statement of relationship 460 



II. Consideration of certain North American forms 460 



J. Remarks on isolation and its relation to speciation 462 



I. Consideration of some of the evidence 462 



1. Evidence from the study of beavers 462 



2. Evidence from certain other families of mammals 465 



( 1 ) Soricidae 465 



(2) Procyonidae 466 



( 3 ) Mustelidae 467 



( 4 ) Muridae 469 



( 5 ) Zapodidae 470 



(6) Aplodontiidae 470 



(7) Ochotonidae 471 



( S ) Cervidae 471 



(9) Summarization 472 



a. Relation of the evidence to some current theo- 

 ries 473 



6. Relation of the evidence to Wagner's theory of 



migration and geographical isolation 475 



c. How have different ecologic niches been filled?.... 479 

 (1. Tentative suggestions regarding the manner in 

 which geographic isolation acts in the process 



of speciation 4S2 



K. Summary 437 



L. Literature cited 490 



INTRODUCTION 



EVOLUTIONARY CONSIDERATIONS 



Problems concerning the causes and conditions of organic evolu- 

 tion are numerous and many of them are as yet unsolved. Even a 

 cursory examination of zoological literature for a number of years 

 reveals the fact that one branch of investigation has held the center 

 of the stage for a time, only to be displaced by another and this in 

 turn by another. This tendency toward successive popularity of 

 different fields of work is for the most part good, of course, since it 



