1916] Taylor: Beavers of Western North America 487 



the differentiation conditioned by isolation is, within limits, cumu- 

 lative. 



As remarked previously, these conclusions are drawn up simply 

 as a possible means of assisting in understanding the enormous 

 and complicated body of facts concerning the speciation of the 

 higher vertebrate animals, and may or may not hold for other 

 classes of organisms. 



Summary 



1. The ranges of seven recognizably differentiated subspecies 

 of beavers touch upon, or lie close to, the Pacific Ocean in western 

 North America. 



2. Since beavers grow throughout their life, the consideration 

 of changes in form, outline, and dimensions of the various parts due 

 to difference in age becomes especially important. There is little 

 change in coloration with age. 



3. The beavers possess a highly specialized dentition, as is shown 

 by the following facts: (1) the permanent dentition exhibits a high 

 degree of hypsodonty, though the milk teeth are brachydont; 



(2) the enamel-fold pattern of the cheek-teeth is much complicated; 



(3) the dental formula is much reduced. The generalized mam- 

 malian formula is If, C\, Pf, Mf X2 = 44, while that of the 

 beaver is I \ , C -» , P > , M|X2 = 20. During ontogeny, the contin- 

 uously growing cheek-teeth first increase in size, then decrease 

 slightly. There are no important differences due to age in the pro- 

 portional relation of the antero-posterior to the transverse diameter 

 of the teeth. The dental armature is a cutting and grinding agency 

 of high efficiency. 



4. The similarly rounded instead of plane conformation of the 

 palato-masillary region, and the similar complication of the enamel 

 pattern of the cheek-teeth in the Sciuromorph genus Castor and in the 

 Hystricomorph genus Erethizon appear to be parallel adaptations: 

 the first condition being an adaptation to the stripping of the bark 

 from twigs and branches, the second to the effective mastication of 

 this food. 



5. Certain readjustments of current systematic conceptions of 

 beaver status are necessary : 



(1) The race of beaver inhabiting the Cook Inlet region of 

 Alaska and ranging south into northern and central British Colum- 



