ANIMALS AND THEIR ENVIKOXMENTS ADAMS. 521 



only approximately. Here also, as in the laboratory, tliey represent 

 a condition of relative eqiullhrium. The laws of the transformation 

 and development of optima are of great ecological importance, as I 

 pointed out several years ago (lOOl). In field study probably the 

 most valuable criterion to be used in the recognition of ecological 

 optima is the normal relative abundance and infl.uence of animals in 

 their breeding environment. 



In the preceding discussion no special emphasis has been placed 

 upon the time element, or the rate at which changes may take place. 

 Natural environments are complexes, in the composition of which 

 several factors are involved. This being true, it is desirable to recall 

 the fact that the ra/e of change is determined by the pace of the 

 slowest factor, or, as Blackman (1905 :289) has expressed it: 



When a process is conditioned as to its rapidity by a number of separate 

 factors, tlie rate of ttie process is limited by tlie pace of the "slowest" factor. 



This is a general law and applies to all changes, internal as well 

 as environmental. 



In closing this section, I wish to call attention to another conclu- 

 sion of the English plant physiologists Blackman and Smith. They 

 state (1911) that from experimental study of the assimilation of 

 w\ater plants, the conception of the optima is untenable, and that the 

 phenomena are better explained as the result of "interacting limiting 

 factors than by the conception of optima" (p. 41-2). This principle 

 is formulated as follows (p. 397) : 



When several factors are possibly controlling a function, a small increase 

 or decrease of the factor that is limiting, and of that factor only, vnIU bring 

 about an alternation of the magnitude of the functional activity. 



It will be of much importance to test the application of this idea 

 to animal responses. ^ 



4. DETEKMINATION OF DYNAMIC STATUS. 



In any study of the energetics of organisms it is desirable to have 

 clearly in mind one of the fundamental conceptions of this science — 

 the dynamic status. The law of conservation of energy teaches us 

 that energy can not be destroyed; that it is transformed only, and 

 thus undergoes a cycle of changes. The animal or an animal com- 

 munity, as a unit and as an agent or transformer, is constantly 

 transforming energy, setting it free. In this sense it originates, but 

 not at a uniform rate. At one time much energy may be transformed 

 and at another very little. When a great amount of energy is being 

 set free, when the animal or community is exerting much influence, 

 we may look upon it as producing pressure or strain. A condition 



1 See also my paper, Migration as a Factor in Evolution : Its Ecological Dynamics. 

 American Naturalist, vol. 52, pp. 4G5-490, 1918 ; vol. 53, pp. 55-78, 1919, for additional 

 reasons for discarding the conception of optima. 



