PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 379 



a nortliorn origin; and the fact that some are circumpohir in their 

 distribution while most of the others (especially among the mam- 

 mals) have congeneric Old World allies further strengthens the 

 theory of their northern origin. Xot only do individuals of the 

 same species increase in size toward the north, but the same is true 

 of the species of the different genera. Again, in the exceptional cases 

 of increase in size soutliAvard, the species belong to southern types, or, 

 more correctly, to types having their center of development within or 

 i]ear the intertropical regions, wdiere occur not only the greatest num- 

 ber 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 Canidse (avoIvcs and foxes), 

 the Felid.T (lynxes and wild-cats), and the Procyonida^ (raccoons). 

 The first tAvo are to some extent cosmopolitan, while the third is 

 strictly American. The Canidse have their largest specific represen- 

 tatives the world over, in the temperate or colder latitudes. In 

 North America the family is represented l)v six species," the smallest 

 of which, speaking generally, are southern and the largest northern. 

 Four of them are among the most Avidely distributed of North 

 American mammals, two, the gray wolf and the common fox, being 

 circumpolar species; another, the Arctic fox, is also circumpolar, but 

 is confined to high latitudes. The three widest-ranging species — the 

 gray wolf, the connnon fox, and the gray fox — are those which pre- 

 sent the most marked variation in size. Taking the skidl as the 

 basis of comparison, it is found that the common Avolf is fully one- 

 fifth larger in the northern parts of British America and Alaska 

 than it is in northern Mexico, where it finds the southern limit of 

 its habitat. Between the largest northern skull and the largest 

 southern skull there is a difference of ahoiit 35 per cent of the mean 

 size! Specimens from the intermediate region show a gradual inter- 

 gradation between these extremes, although many of the examples 

 from the upper INlissouri country" are nearly as large as those from 

 the extreme north. 



The common fox, though occurring as far north as the wolf, is 

 nnich more restricted in its southward range, especially along the 

 Atlantic coast, and presents a correspondingly smaller amount of 

 variation in size. The Alaskan animal, however, averages about one- 

 tenth larger than the average size of specimens from New England. 



oThe gray wolf {Canis lupiifi [=C. ooeidentaliH and allied forms]), the 

 prairie wolf (('. latrans [now treated as a sronp of a dozen or more closely 

 related species and subspecies]), the Arctic fox (VnlpcH hifjopii.s [now sepa- 

 rated into several forms]), the common fox (V. nlopex [ = T'. fulvus and numer- 

 ous related forms] ), the kit fox (F. velox [now subdivided into several forms] ). 

 and the gray fox (JJrocyon virginianiis [ = U. cinereoargcntem, with a dozen or 

 more subspecies]). 



