1916] Taylor: Beavers of Western North America 479 



It is noteworthy that in all these critical cases there is not a 

 sufficient quantity of fossil and Recent material for the adequate 

 elucidation of their relationships. This is especially apparent with 

 the weasels, skunks, and aplodonts just cited. On the other hand, 

 in cases where material is abundant and the status of the forms is 

 reasonably well established, the great mass of facts of distribution 

 harmonizes with Wagner's theory. 



How Have Different Ecologic Niches Been Filled? 



It should be remembered, of course, that had living forms not 

 possessed the power of adapting themselves or becoming adapted to 

 different ecologic niches we would not have had the various niches 

 tilled. But the evidence from mammals and higher vertebrates 

 would seem to indicate that geographic isolation is prerequisite to 

 any kind of differentiation ; physiological isolation, or practical ster- 

 ility, being gradually assumed in proportion as the completeness 

 and long continuance of geographical isolation, and the association 

 of it with diversity of external conditions, permit or condition 

 morphological changes extending to the reproductive system. The 

 broader the distribution of the species and subspecies of any group 

 of mammals or higher vertebrates, the greater the probability that 

 some of the forms will find themselves in ecologic niches which, 

 while they resemble in a general way the original group niche, will 

 still differ from it in some important particulars. In strict terms, 

 no two ecologic niches, situated in different geographic areas, can be 

 precisely the same. 



Thus it may be noted that while the so-called pa nam in tin us 

 group of the genus Perognathus occupies the same general ecologic 

 niche wherever found, as a matter of fact, the particular ecologic 

 niche occupied by each species or subspecies is somewhat different 

 from that occupied by any of the other species or subspecies. 

 Grinnell and Swarth (1913, pp. 390, 392) have listed in their 

 "fourth category", Table D, a number of forms of birds and mam- 

 mals of which also this may be said to be true, and in the opinion 

 of the writer study of group distribution in vertebrates shows this 

 to be a rule of practically universal applicability. 



The status of things may be graphically represented by a diagram, 

 as in fig. Q. Let the capital letter stand for the stock or species, the 

 lower-case letter represent the ecologic niche it occupies, and the 

 arabic numeral the geographic area in which the ecologic niche is 



