472 University of California Publications in Zoology [Vol.12 



Summarization 



Having now adduced evidence from a study of nine families of 

 mammals, representative of four orders, we may profitably undertake 

 to discover whither it leads us. 



Before proceeding it ought to be stated that this evidence is typi- 

 cal of that which would be gathered in course of the consideration of 

 any family whatever of California mammals. The evidence might be 

 greatly extended, but such extension would imply much unjustified 

 repetition, for further evidence would probably not illustrate the 

 actual state of things any better than that which we now have at 

 hand. 



It should be remembered that our present concern is the gather- 

 ing, so far as that may be here practicable, of the evidence on the 

 differentiation of species, or polytypic evolution (the splitting up of a 

 parent stock into a number of differentiated stocks), as opposed to 

 monotypic evolution (continuous change through time and in a given 

 direction of the entire stock without divarication). Thus we are not 

 discussing the problem of the mode of organic evolution primarily, 

 except in so far as this problem is bound up with considerations of 

 differentiation. 



Furthermore, we propose to discuss this matter from the stand- 

 point of studies, first of mammals, and second, of the higher verte- 

 brates in general. It is hardly to be assumed that the same con- 

 ditions and factors of organic evolution apply universally, or that, 

 where they do apply, they must act in exactly the same way. 



An illustration of the heterogeneity of the organic world is for- 

 tunately furnished by the very problem with which we are here deal- 

 ing, namely, that of the place of geographic isolation in specific dif- 

 ferentiation. "We are indebted to Kofoid (1907a, pp. 500-506) for 

 calling pointed attention to the possible limitations of isolation in the 

 origin of species. After remarking the high degree of cosmopoli- 

 tanism of the fresh-water microfauna and flora, Kofoid emphasizes 

 the fact that there is not only extensive coincident distribution of 

 related species, but that there are numerous cases in which there is 

 actual coexistence of the most closely related species in the same 

 habitat. 



Similar evidence is adduced by Clark (1911, p. 23), who demon- 

 strates the coincident distribution of closely related species among 

 the Ophiurans. There seems to be abundant evidence that very 

 closely related species of this group, often the most closely related, 



