

1916] Taylor: Beavers of Western North America 477 



maniculatus oreas, are rather exceptional. Concerning the first 

 case Osgood says (1909, pp. 69, 70) : 



The transition from ijambeU to rubidus along the line between their ranges 

 is rather sudden, suggesting the possibility of hybridizing. Prom several 

 localities specimens fairly typical of both forms are known, from others we 

 have both forms and apparent intermediates, and from still others all speci- 

 mens thus far obtained are intermediate not typical of either form. This is 

 exactly what would be expected upon the theory of hybridism, but of course 

 it cannot be considered as conclusive proof. 



Concerning the second he says (pp. 52, 53) : 



The case is very similar to that of gambeli and rubidus in California, the 

 complications of which may be due either to hybridization or to intergrada- 

 tion. It is already known that oreas and austerus occur together at a number 

 of localities and apparently maintain their respective characters. At other 

 places only one form has thus far been found, at others extremes of both 

 forms and intermediates occur, and at still others intermediates only. There 

 is no environmental distinction as in the case of gambeli and rubidus, for 

 oreas and austerus live under apparently identical conditions. Although only 

 one form has been found at the respective type localities of oreas and 

 austerus, both occur together near by, and further collecting may show that 

 they do so over a wide area. Specimens which appear to be intermediate 

 between oreas and austerus may in reality represent special differentiations 

 of the one or the other showing accidental parallelism. 



These cases are very puzzling, as the pairs of species mentioned 

 are very closely if not most closely related to each other, and on 

 Wagner's hypothesis should not occur together. Note should be 

 made of at least two possible explanations: It may be that, having 

 originated in different geographic areas, and having undergone some 

 degree of differentiation, there has been re-invasion, and that, even 

 though very closely related, they occupy slightly different ecologic 

 niches, and are able to coexist in the same place because of some 

 measure of physiological isolation. Or it may be that we have here 

 an illustration of Mendelian inheritance in nature, the two sub- 

 species crossing freely. The extreme rarity among higher verte- 

 brates of this latter type of behavior where ranges overlap at their 

 margins, would seem to constitute an argument for the first, rather 

 than the second explanation, but the case is not entirely clear. 



It will be remembered that Mustela arizonensis is found in the 

 Transition and Boreal zones of the Sierra Nevada and the Rocky 

 Mountain systems. On the Cascades and the Trinity Mountains 

 its place is taken by Mustela saturata, a closely related form. 

 Between the Sierra Nevada and the Rockv Mountains there would 



