SONORAN REGION 197 



abandoning faunistic regions in a work of this kind, and I 

 feel there is little advantage in discussing the merits of one 

 regional system above another. I only wish to direct attention 

 to the fact that the distinctness and importance of this south- 

 western part of North America had long ago been recognised 

 by zoologists. Professor Carpenter urges that two distinct 

 faunas exist in America. I can trace even more than two, 

 for a South American element is very prominently diffused 

 throughout a large portion of the States. I cannot concur 

 at all in Dr. Merriam's view* that "except for the. 

 presence, chiefly in the southern United States, of a compara- 

 tively few forms derived from the tropical region, the fauna 

 and flora of North America are as distinctive and indepen- 

 dent of the existence of this area as if separated from it by 

 the broad ocean." To place ourselves within fixed and 

 strictly limited boundaries at all seems to me a mistake. I 

 do not limit myself in any way to political frontiers, and 

 if I had set myself a boundary, I should: have been inclined to 

 place it across the isthmus of Tehuantepec. At this point 

 North America was evidently separated for some time from; 

 Central America by a marine channel, though this division 

 was not so effectual in keeping two great faunas distinct as 

 one might expect. All this, however, will be discussed at 

 greater length when we come to deal with the fauna of Central 

 America. 



Although vast tracts of south-western North America are 

 nothing more than deserts, there is, on the whole, an extra- 

 ordinary abundance and variety of animal life. I have 

 repeatedly pointed out in previous chapters that the roots 

 of certain groups of eastern animals must be looked for in 

 the south-west or west. In drawing attention to the strange 

 affinities of some apparently very ancient east-American 

 forms, such as the smooth and the rough green snakes, and 

 east-Asiatic snakes, I urged that they must originally have 

 spread eastward from south-western North America (p. 125). 

 Among the tortoises, too, certain eastern groups can be 

 traced to a remote western origin, although no longer, 

 resident there. Thus there has arisen a comparatively modern 

 repetition of that dissimilarity between the eastern a,nd 



* Merriam, 0. H., " Distribution of Life in North America,'' p. 37. 



