DISTRIBUTION OF TERRESTRIAL VERTEBRATES 91 



great interest and enormous theoretical importance since the time of 

 Darwin, a satisfactory evaluation of the role of competition in the 

 determination of distribution in general is obviously impossible at 

 the present time. As Hutchinson (1957, p. 419), has pointed out: 



The only conclusion that one can draw at the present from the observa- 

 tions is that although animal communities appear qualitatively to be 

 constructed as if competition were regulating their structure, even in the 

 best studied cases there are nearly always difficulties and unexplored 

 possibilities. These difficulties suggest that if competition is determinative 

 it either acts intermittently ... or it is a more subtle process than has 

 been supposed. 



My obvious reluctance to accept the direct role of physiology in 

 the determination of distribution of species probably needs defense 

 from the students of geographic variation in western North America, 

 where so many subspecies and species have been shown to have 

 limits that can be readily correlated with vegetation types and hence 

 indirectly with conditions of the physical environment. Since such 

 correlations are striking, it is often assumed that the distributional 

 limits are physiologically determined and that closely related species 

 or even subspecies are characterized by physiological differences. 

 There is little a priori reason to presume that animals are any less 

 variable in physiology than in morphology. Aside from coloration, 

 however, the minute morphological differences separating subspecies 

 or closely related species are not necessarily adaptive; similarly, 

 small physiological differences between closely related forms need 

 not be adaptive. If physiological differences are not adaptive, they 

 have little significance in determining the distribution of the forms 

 that possess them. Moreover, when one deals with the smallest 

 taxonomic categories — subspecies and obscurely delimited species — 

 and finds adaptive physiological differences, it is impossible to state 

 categorically whether or not these differences allow or follow 

 changes in distribution. 



In view of the difficulty of demonstrating physiologically deter- 

 mined distributional limits and physiologically determined competi- 

 tive success in terrestrial vertebrates, it is reasonable to turn to 

 habitat selection and ecological tolerance for help in understanding 

 distribution. The correlation of habitat with both local and general 

 distribution is familiar to all field zoologists. It is a well-documented 

 fact that discontinuities of populations of terrestrial vertebrates 



