524 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



The responses of animals to the conditions in which they live are 

 of a composite character. Certain responses, such as the chirping re- 

 sponse of a coot within the egg, are inherited and are relatively auto- 

 matic in character ; others are greatly modified by experience, as when 

 an animal " learns," or forms a habit by repeated responses. 



The responses of animals to the conditions of existence are the 

 basis for any study of their relations, not only to other members of 

 their own species, but to all elements, living or otherwise, of their 

 complete environment. It is from this standpoint that animals must 

 be considered in estimating their place in the economy of nature ; that 

 is, in estimating how they influence one another in an association of 

 animals living together in the same habitat, and in judging of their 

 relation to the succession of animal communities, and even to man 

 himself. 



G. THE INTERRELATION OF ANIMALS. 



A group or association of animals or plants is like a single organism in the 

 fact that it hrings to bear upon the outer world only the surplus of forces re- 

 maining after all conflicts interior to itself have heen adjusted. Whatever 

 expenditure of energy Is necessary to maintain the existing internal balance 

 amounts to so much power locked up and rendered unavailable for external 

 use. — S. A. Forbes. 



We have now seen the dependence of the animal upon its environ- 

 ment, as this forms the basis for an understanding of conditions in- 

 volved in the problem of maintenance or the upkeep of the animal. 

 The optimum conditions for prolonged maintenance produce the vital 

 and ecological optima. These conditions imply more than mere 

 maintenance; they mean as well, a degree of favorable conditions 

 which permits the animal to exert an influence or stress upon its en- 

 vironment. As Forbes has said, if all the energy available to the 

 animal is utilized internally there will be nothing left to influence 

 the environment. Metabolic changes show that large amounts of 

 energy and substance are used in maintenance. Under optimum 

 conditions even greater amounts must exist. An animal must not 

 only be able to maintain itself against other kinds of animals but 

 even against its own kind, for the overproduction of its own race will 

 be practicalty self-destructive. A good example of this kind of in- 

 fluence is seen in the hordes of lemmings which migrate, even into 

 the sea, when overproduction becomes extreme. 



The vital and ecological optima are thus to be looked upon as in- 

 ternally balanced, but externally, not as a state .of balance or poise, but 

 as a condition in which the animal is exerting stress, pressure, or in- 

 fluence upon its environment, instead of being passive or inert. A 

 group of animals living together in any given condition such as an 

 association, is an assemblage of interacting organisms. The active, 

 free-moving animals collide with each other, with other kinds of ani- 



