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 front 
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 and 
* Merriam, C. H., “ Distribution of Life in North America,’’ p. 37. 
