DIRECT EQUILIBRATION. 523 



adaptive and which are not? How are we to distinguish 

 between them? 



There can be no direct equilibration with an external 

 agency which, if it acts at all, acts fatally; since the organism 

 to be adapted disappears. Conversely, some inaccessible 

 benefit which a small modification in the organism would 

 make accessible, cannot by its action tend to produce this 

 modification: the modification and the benefit do not stand 

 in dynamic relation. The only new incident forces which 

 can work the changes of function and structure required to 

 bring any animal or plant into equilibrium with them, are 

 such incident forces as operate on this animal or plant, 

 either continuously or frequently. They must be capable of 

 appreciably changing that set of complex rhythmical actions 

 and reactions constituting the life of the organism; and yet 

 must not usually produce perturbations that are fatal. Let 

 us see what are the limits to direct equilibration hence 

 arising. 



§ 161. In plants, organs engaged in nutrition, and exposed 

 to variations in the amounts and proportions of matters and 

 forces utilized in nutrition, may be expected to undergo cor- 

 responding variations. We find evidence that they do this. 

 The " changes of habit " which are common in plants, when 

 taken to places unlike in climate or soil to those before in- 

 habited by them, are changes of parts in which the modified 

 external actions directly produce modified internal actions. 

 The characters of the stem and shoots as woody or succulent, 

 erect or procumbent; of the leaves in respect of their sizes, 

 thicknesses, and textures; of the roots in their degrees of 

 development and modes of growth; are obviously in imme- 

 diate relation to the characters of the environment. A per- 

 manent difference in the quantity of light or heat affects, day 

 after day, the processes going on in the leaves. Habitual rain 

 or drought alters all the assimilative actions, and appre- 

 ciably influences the organs that carry them on. Some par- 



