464 University of California Publications in Zoology [Vol- 12 



well as broad deserts on the east and south. The two subspecies of 

 subauratus, namely subauratus and shastensis, are much more closely 

 related to each other than is either to any other known beaver in the 

 world. Although they are inhabitants of the same hydrographic 

 basin, they are efficiently separated by distance in combination with 

 the "narrows", by way of which the Pit River cuts through the 

 Sierra Nevada Mountains. 



It must be immediately apparent that these facts, so far as they 

 go, fulfill the requirements of Wagner and Jordan's law : "Given any 

 species in any region, the nearest related species is not likely to be 

 found in the same region nor in a remote region, but in a neighboring 

 district separated from the first by a barrier of some sort. ' ' 



It is now quite well established that on continuous laud areas 

 temperature is the most efficient of all barriers, with humidity of the 

 atmosphere a close second. It is a remarkable fact that the genus 

 Castor ranges, undergoing at the same time but little change, through 

 all the life-zones (based on temperature) from the Hudsonian of the 

 northern limit of trees to the Lower Sonoran of the southern deserts, 

 and that it is found in faunal areas (based on humidity) as widely 

 different as the Sitkan district of southeastern Alaska and the Colo- 

 rado Desert of the southwestern United States. Although the semi- 

 aquatic environment of the beaver is doubtless more uniform through- 

 out this great range of temperature and moisture conditions than is 

 the typical terrestrial environment, it must still be conceded that the 

 genus Castor is subjected to very different environments in different 

 parts of its range. The writer at this moment finds it impossible to 

 assign any adaptive significance to the subspecific and specific differ- 

 entiatory cranial characters of beavers. It must be confessed that 

 the maintenance by the beaver of its chief characteristics through a 

 very wide range of environmental conditions, coupled with the further 

 fact that it is difficult to attribute any adaptive significance to the 

 cranial specific and subspecific characters, invite one to the hypothesis 

 that these characteristics of the different races of beaver are due to a 

 cumulation of what are for the most part inutile characters through 

 the fact of the geographical isolation, alone, of the various beaver 

 stocks. 



An alternative view is, of course, that our inability to see the 

 adaptive significance of these differentiatory characters, or definitely 

 to correlate them with characters which are obviously adaptive, merely 

 testifies to the limitations of our own knowledge, and not at all that 



