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THE AMERICAN NATURALIST [Vol. LIII 



must be, (a) of a magnitude sufficient to overcome the 

 inertia of the system, that is it must be a liminal stimulus 

 and (h) it must relate to a limiting factor of assimila- 

 tion) cf. Hooker, '17, p. 204). Furthermore, the change 

 in the relation between the organism and a factor of its 

 environment may be produced either by a change in the 

 environment, i. e., it is heterotrophic, or by a change in 

 the organism, i. e., it is autotrophic, or by changes in 

 both. 



The response is characterized by the facts that {a) its 

 nature and direction are determined by the stimulus but 

 {h) the energy is supplied by processes that occur within 

 the system; in other words the stimulus releases the 

 response. Thus in the ice-water-vapor system when heat 

 is withdrawn, this is supplied by the latent heat of fusion 

 of ice. In this simple system there is a direct relation 

 between the amount of heat withdrawn and the amount 

 supplied. Such a relation does not exist in the behavior 

 of organisms, nor should we expect it in such complicated 

 systems. A wealth of examples illustrating the applica- 

 tion of the theorem of Le Chatelier to the behavior of 

 plants and animals will be found in Bancroft's article 

 ('17). A brief discussion of the integrating character 

 of development, evolution and biotic succession has been 

 made elsewhere (Hooker, '17) and supplies illustrations 

 of these principles. It will be sufficient to reiterate here 

 that cells, organs and groups of organisms form systems 

 as well as the single organism. Correlations and mor- 

 phogenic responses are therefore conditioned in accord- 

 ance with the theorem of Le Chatelier, and it is unneces- 

 sary to postulate the existence of hypothetical inhibiting 

 substances to account for tlio normal behavior of parts.' 



Inasmuch as tlic I'cactinn of s>-st(']n is directed ac- 

 cording to the tiicoiTin of Cliatcru'i-, i-xwy system in 

 equilibrium is tclroloo-ieal. The means that i)roduce the 

 reaction are directed to a definite end, to overcome the 

 constraint, and the i-eaetion might be said to take place 

 in order that the system may be preserved. This is evi- 



