13 



and ecological optima. These conditions imply more than mere main- 

 tenance; they mean as well, a degree of favorable conditions which 

 permits the animal to exert an influence or stress upon its environment. 

 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 practically self-destructive. 

 A good example of this kind of influence is seen in the hordes of lem- 

 mings 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- 

 mals, especially the relatively sedentary kinds, and with their environ- 

 ment of plants and the inorganic factors. The relatively sedentary 

 animals are correspondingly bombarded by all elements of their en- 

 vironment. The association, as a whole, is thus in a continuous proc- 

 ess of bombardment and response from every possible angle, and just 

 as the individual animal is stimulated and responds, so all the mem- 

 bers of any association are stimulated and respond in a similar man- 

 ner. It is by this form of activity that animals not only maintain 

 themselves but exert a radiating influence. 



It will assist in realizing the constant pressure exerted by animals 

 if we compare their activity to the flow of a stream. The pressure ex- 

 erted by the stream may be realized if by a dam or similar means the 

 current is resisted. Think for a moment of the amount of energy 

 which would be transformed in an effort to prevent animals (or 

 plants) from taking possession of a favorable habitat. Imagine an 

 area lo feet square and think of the effort it would require to prevent 

 animals permanently from invading and establishing themselves in this 

 habitat if no barriers were interposed, and if the means of destruction 

 of the invaders were not so drastic that they materially changed the 

 character of the habitat. Increase the size of the area and the diffi- 

 culties will increase in geometrical ratio, and the utter futility of such 

 an undertaking will soon be realized. The spreading processes of the 

 gypsy moth in Massachusetts, and of tiie San Jose scale and the cotton 



