48 PRIMARY FACTORS OF ORGASUC EVOLUTION. 



ical regions. This may be illustrated by reference to 

 the distribution of the higher classes of vertebrates in 

 North America. Among the species occurring north of 

 Mexico there are very few that may not be supposed to 

 have had a northern origin ; and the fact that some 

 are circumpolar in their distribution, w^hile most of the 

 others (especially among the mammals) have congeneric 

 Old World allies further strengthens the theory of their 

 northern origin. Not only do individuals of the same 

 species increase in size toward the north, but the same 

 is true of the species of different genera. Again, in 

 the exceptional cases of increase in size southward, the 

 species belong to southern types, or, more correctly, 

 to types having their center of development within or 

 near the intertropical regions, where occur, not only the 

 greatest number of the specific representatives of the 

 type, but also the largest. 



"For more detailed illustration we may take three 

 families of the North American Carnivora ; namely, 

 the Canidae (wolves and foxes), the Felidae (lynxes 

 and wild cats), and the Procyonidae (raccoons). The 

 first two are to some extent cosmopolitan, while the 

 third is strictly American. The Canidae have their 

 largest specific representatives, the world over, in the 

 temperate or colder latitudes. In North America the 

 family is represented by six species, ^ the smallest of 

 which (speaking generally) are southern, and the larg- 

 est northern. Four of them are among the most widely 

 distributed of North American mammals, two (the 

 gray wolf and the common fox) being circumpolar spe- 

 cies ; another (the Arctic fox) is also circumpolar, but 



1 The gray wolf (Canis lupus), the coyote (C lairans], the Arctic fox [I'ul- 

 pcslagopus), the coininon fox {/'. alcpcx), the kit fox (/'. velox), and the gray 

 fox (?'. cinereoargentatus). 



