144 



THE AMERICAN NATURALIST [VOL.XLVHI 



TABLE IX 



Speciation of Amphibia and Reptilia in North American Areas 

 Data from Cope (1889), (1898) 

















Sp. 







Amphibia 

 Ophidia..'. 





143 



45 



4.64 



The "Lower California district" consists of only the 

 tip of Lower California; the "Western subregion" em- 

 braces the Pacific slope of North America from Northern 

 Mexico, east of the Sierras, to Oregon, where it crosses 

 the Sierras to the Rocky Mountains, including northern 

 Idaho, eastern Montana, and most of British Columbia. 

 The " Medicolumbian region" includes northern and cen- 

 tral Mexico, and most of the United States and Canada 

 north to a line drawn diagonally from New England to 

 Alaska, interdigitating on its border with the "Holarctic 

 region." 



The almost exactly parallel increase in the indices of 

 modification in the three groups of cold-blooded verte- 

 brates considered, as the area is extended, is quite remark- 

 able. All three groups average from 1.25 to 1.33 species 

 per genus in the smallest area, from 2.15 to 2.30 in the 

 intermediate area, and from 4.24 to 4.64 in the largest 



As suggested by Professor Kofoid, a factor influencing 

 speciation in such diverse vertebrates as mammals, birds, 

 reptiles, and amphibians, should be very widely appli- 

 cable to speciation in the entire animal kingdom. 



A series of statistics relating to various orders of in- 

 sects and other invertebrates has been compiled to ascer- 

 tain whether in these groups as well as in vertebrates, the 

 number of species increases out of proportion to the 

 genera, as the size of the area, in a distributional sense, 

 is enlarged. 



