DISTRIBUTION OF TERRESTRIAL VERTEBRATES 85 



logical capacities of living forms, it is possible to adduce some obvi- 

 ous, familiar, and general statements about the broad relations of 

 physiology to the distribution of terrestrial vertebrates. Such an 

 effort is greatly facilitated by application of Liebig's law of the 

 minimum (Hesse, Allee, and Schmidt, 1951, p. 26), one of the most 

 useful of generalizations to which an ecological physiologist can turn. 

 In the context of the present essay this generalization may be stated 

 thus : the distribution of a species will be controlled by that environ- 

 mental factor for which it has the narrowest range of adaptability or 

 control. The limiting factors will of course be different at different 

 stages in the life cycle and will vary from group to group and from 

 time to time. 



The evolution of terrestrial vertebrates has been characterized 

 first, by increasingly effective homeostatic mechanisms and second, 

 by increasingly variable and flexible behavior. Together these two 

 trends mean that the evolutionary history of vertebrates has resulted 

 in increasing physiological competence and, at the same time, 

 increasing capacity to select from the environment the special physi- 

 cal situations that are appropriate to an animal's physiological 

 capacity. This increase in ecological versatility allows some forms 

 to occupy a remarkably diverse array of habitats and makes the 

 determination of distributionally limiting factors an intriguingly 

 subtle problem. If we examine the major groups of terrestrial verte- 

 brates with regard to the aspects of their physiology that are likely 

 to be limiting, we can make several obvious general observations. 



Amphibians. Amphibia show poor osmoregulation (see Sawyer, 

 1956, for a recent review), poor control of water loss (Cohen, 1952; 

 Thorson, 1956), and complete lack of physiological thermoregulation 

 other than the passive cooling incidental to dehydration. 



Reptiles. Of the major homeostatic capacities reptiles lack only 

 effective physiological thermoregulation, and they compensate for 

 this with surprisingly effective behavioral thermoregulation (Cowles 

 and Bogert, 1944; Bogert, 1949; Norris, 1953). A dramatic example 

 of behavioral thermoregulation is shown by the Andean lizard 

 Liolaemus multiformis, which under some circumstances can achieve 

 body temperatures as much as 30°C above air temperature (Pear- 

 son, 1954a). 



Birds. Birds have reached a level of homeostatic control com- 

 parable to that of mammals. Considering their small size, they have 



