DISTRIBUTION OF TERRESTRIAL VERTEBRATES 83 



broad that It is of little assistance In the analysis of any specific 

 distribution. The aspects of physiology that are directly pertinent 

 to distribution — and these are the only ones with which it Is ordi- 

 narily possible to deal — are not those concerned with internal inte- 

 gration, but are those that cither involve the animal's exchanges 

 with the environment or control and regulate these exchanges, and 

 those that affect reproductive performance. The exchanges referred 

 to involve food, water, heat, radiation, and metabolic wastes, and 

 are usually studied in terms of rates and limits. The factors con- 

 trolling reproductive performance under natural conditions are at 

 present only in the first stages of analysis (Lack, 1954) and offer a 

 challenging series of problems related to distribution. 



Since an organism is Inseparable from its environment, any person 

 who attempts to understand an organism's distribution must keep 

 constantly in mind that the item being studied is neither a stuffed 

 skin, a pickled specimen, nor a dot on a map. It is not even the live 

 organism held In the hand, caged in the laboratory, or seen in the 

 field. It is a complex Interaction between a self-sustaining physico- 

 chemical system and the environment. An obvious corollary Is that 

 to know the organism it is necessary to know its environment. If 

 this view is valid, and if the distributlonally relevant data of physiol- 

 ogy relate to the dynamics of the complex Interaction between 

 organism and environment, then to evaluate the contribution of 

 physiological data to knowledge of distribution, one must first 

 examine the environment critically, analytically, and in detail. 

 Obviously a searching examination of the environment cannot be 

 made In a general discussion addressed to only the broad aspects of 

 the problem, but for purposes of comparison we may consider the 

 sea. From the point of view of this discussion, the most conspicuous 

 feature of the open sea is that it offers few places for an animal to go 

 to avoid unfavorable conditions that may develop locally. If one 

 excludes shore and estuarlne areas, the number of aquatic micro- 

 habitats that offer significant possibilities of escape from unfavorable 

 conditions is negligible. We can therefore expect that marine organ- 

 isms will often be limited in distribution by physical conditions, 

 particularly temperature. In contrast, the terrestrial environment 

 and to a lesser degree the freshwater habitat offer many mlcrohabl- 

 tats that make available an enormous range of temperature, mois- 

 ture, and radiation. 



