9 



increase or a decrease, are departures froju the most favorable condi- 

 tions toward less favorable conditions, and hence toward limiting con- 

 ditions. This form of expression is mainly that of the laboratory ; it 

 is desirable therefore, in addition, to express it in terms of the normal 

 habitat. In nature we look upon the optimum as that complex of 

 habitat factors which is the most favorable, and departure in any di- 

 rection from this optimum intensity is in the direction of a less favor- 

 able degree of intensity or into unfavorable conditions. From this 

 standpoint any unfavorable condition is a liniiting factor and may re- 

 tard, hasten, or prevent vital and ecological activities. Optima arc 

 thus almost ideal conditions, and are probably realized in nature only 

 to a limited degree ; in other words only approximately. Here also, 

 as in the laboratory, they represent a condition of relative cquilibriuni. 

 The laws of the transformation and development of optima are of 

 great ecological importance, as I pointed out several years ago ('04). 

 In field study probably the most valuable criterion to be used in the 

 recognition of ecological optima is the normal relative abundance and 

 influence 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 sev- 

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

 fact that the rate of change is determined by the pace of the slowest 

 factor, or, as Blackman ('05:289) has expressed it: "When a proc- 

 ess is conditioned as to its rapidity by a number of separate factors, 

 the rate of the process is limited by the 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 ('11) that from experimental study of the assimilation of water 

 plants, the conception of the optima is untenable, and that the phe- 

 nomena are better explained as the result of "interacting limiting 

 factors than by the conception of optima" (p. 412). 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, will bring about an alternation of 

 the magnitude of the functional activity." It will be of much impor- 

 tance to test the application of this idea to animal responses. 



4. de;termination 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 — 



