ENVIRONMENTAL FACTORS 47 



limited by the presence of bushes, since it comes out at night ; 

 but it requires deep sand in order that its burrows may be 

 sufficiently cool during the day. The reaUty of these limiting 

 factors is shown by the fact that both rodents die or are ex- 

 tremely affected if exposed to the sun for any length of time.^'' 



16. In many cases temperature has a direct effect in limiting 

 the activities or in controlling the development of animals. 

 Austin 37 showed that the larvae of the house fly {Musca 

 domestica) died at temperatures of 105° F. or over, and that 

 if piles of horse manure in which there were fly larvae living 

 were close-packed, the heat generated by decay inside the 

 heap was sufficient to kill off the larvae except at the edges, 

 and that they actually migrate away from the hotter parts 

 towards the periphery. Temperature-regulated animals like 

 mammals may also, of course, be severely affected by heat, as 

 shown by the fact that nearly all desert mammals are nocturnal ; 

 while the Alaskan fur seal, which is normally adapted to the 

 cold foggy climate of the Pribiloff Islands, suffers a great deal 

 if the temperature rises above 46° to 48° F. At this tem- 

 perature they show great distress when moving about and 

 fighting, while at 55° to 60° they lie about motionless except 

 that they all fan themselves vigorously with their flippers ."^^ 



17. Since we do not intend in this book to do more than 

 describe certain principles and methods in ecology, illustrated 

 by examples, we need not enumerate any more examples of 

 the action of physical and chemical factors in limiting the 

 distribution of animals. We have not, however, mentioned 

 any cases of biotic limiting factors, and we shall therefore 

 conclude this chapter by mentioning a few out of the enormous 

 number which exist. It is one of the commonest things in 

 nature to find a herbivorous animal which is attached solely to 

 one plant either for food, or for breeding purposes, or for both. 

 It is hardly necessary to quote examples from amongst insects, 

 since they are so numerous. Such Hmited choice of plants for 

 food or nesting is not so commonly found among the higher 

 vertebrates, but there are some interesting examples. The 

 beaver {Castor fiber) used to occur all over North America from 

 the Colorado River and Northern Florida right up to Alaska 



