394 University of California Publications in Zoology [Vol. 10 



within one or two generations. It is urged that should individuals 

 be transplanted, say, from the desert to the San Diegan district, 

 characters of color, quality of pelage or of plumage, size, and 

 proportions similar to the race native in the new locality would 

 promptly be acquired. 



As an argument against this notion of the evanescence of sub- 

 specific characters, we wish to call attention to the numerous 

 cases in Table C, where the ranges of forms extend in one direc- 

 tion or the other over into opposite faunal areas with their 

 sharply contrasted conditions more particularly of humidity. 

 Some of the mammals are of probably slow locomotion as regards 

 home centers; so that many generations must have been sub- 

 jected to the foreign environment to have allowed invasion to 

 the distance attained. And yet in the extreme cases the char- 

 acters of the stuck form have been retained with no perceptible 

 modification — an "experiment" in the laboratory of nature (see 

 also table D). 



As has already been made clear in the discussion on preceding 

 pages, we cannot expect to derive universal laws for the behavior 

 of species, to be applicable uniformly in any region like the San 

 Jacinto area where two faunas meet. Perhaps the only general 

 rule that can be laid down is that there is no c.cact concordance in 

 the distributional behavior of all the animals of a region. 



Upon reflection it is difficult to conceive of precisely the same 

 set of delimiting factors operating upon any two species alike. 

 The condition of diversity is thus explicable, as regards inter- 

 faunal invasion of individual species, degree of blending or dis- 

 tinctness between adjacent representative forms, and restriction 

 versus cosmopolitanism in general distribution. 



Possible generalities are as follows: That the more restricted 

 as to association a form is in its distribution, the more liable is it 

 to manifest the phenomenon of geographic variation. In other 

 words the less adaptable a species the more chance for the action 

 of the factor of isolation which is essential for the multiplication 

 of subspecies, and hence of specific types. Operating to offset the 

 effects of isolation are the processes of hybridization, and its 

 probable climax, intergradation (see pp. 345, 374), which only 

 long distance between differentiation centers can counterbalance 

 (see Grinned, 1904b, p. 372). 



