378 ORGANIC GROWTH 



fication of structure, we mean spontaneous use, the active 

 motion of the organism ; and the influence of exercise in 

 causing growth is therefore principally observed in animals. 

 But when subsequently we consider for a short time the 

 causes of modification in the vegetable kingdom, we shall see 

 clearly how impossible it often is to separate the direct effects 

 of external stimuli from those of exercise, how the two 

 frequently coincide. And from a physical point of view this 

 requires no explanation. 



It is, however, a necessary consequence of my conception 

 of growth that I should regard the effects of indirect stimula- 

 tion of the living substance also as growth. 



