92 G. A. BARTHOLOMEW 



coincide with changes in plant formations and soil types. These 

 discontinuities in distribution are related to active selection of 

 habitat by the animals involved. The ability to recognize and react 

 to factors in the environment, in such a manner that a given species 

 characteristically occupies a certain type of situation, is of course 

 referred to as habitat selection. As Miller (1956, p. 269) has pointed 

 out, "It does not imply selection of a habitat coincident with the 

 limits of environmental tolerance of the species but usually reaction 

 to some feature of the habitat far within those limits . . ." In areas 

 of great altitudinal relief such as western North America, many 

 clear-cut examples of the roles of habitat selection are available 

 (Miller, 1942). A spectacular instance is ofTered in the mountains of 

 northern Nicaragua where tropical rain forests interdigitate with 

 montane pine forests and each vegetational complex supports its 

 own characteristic avifauna, so that boreal species such as the Red 

 Crossbill (Loxia curvirostra) and tropical species such as the Jacamar 

 (Galbula ruficauda) may live only meters apart (T. R. Howell, 

 personal communication). Equally spectacular examples can be 

 cited for other groups. An unusually clear-cut example of the role of 

 the substratum in determining distribution is afforded by the fringe- 

 toed lizards of the iguanid genus Uma of the deserts of southwestern 

 America. Members of this genus occur only on aeolian sand, and the 

 changes in distribution of present day forms are determined by the 

 movements of the sand dunes that they occupy. Despite the total 

 dependence of this genus on a specific and limited physical habitat, 

 it occupies a broad altitudinal zone extending from 244 feet below 

 sea level to 3,700 feet above sea level (Norris, 1958). 



SUMMARY AND CONCLUSIONS 



It is the thesis of this paper that although the distribution of 

 many marine and aquatic organisms and many terrestrial inverte- 

 brates may be explicable in terms of physiological tolerances, no 

 such general statement can at present be made for terrestrial 

 vertebrates. The relationship between physiology and distribution 

 becomes progressively more obscure as one ascends the phylogenetic 

 series of vertebrates. The homeostatic mechanisms of terrestrial 

 vertebrates and the exceedingly complex relations which their 

 behavior allows them to maintain with the physical environment 

 make any assignment of causality between physiology and distribu- 



