PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 749 



similar action of chemical reagents on plant and animal. It 

 must be remembered, moreover, that one of the greatest 

 advantages to be derived from such a use of the plant as 

 a means of physiological investigation, lies in the fact that 

 this study can be carried on with intact and growing 

 specimens under normal circumstances. The experimental 

 conditions are in this case, therefore, better than those which 

 correspond to them in regard to animal tissues, since the 

 latter specimen will generally be found to be suffering from 

 the effects of injury, and may therefore have been rendered 

 abnormal to an unknown degree. The study of the re- 

 sponsive phenomena in plants must thus form an integral part 

 of physiological investigation into the various problems relat- 

 ing to the irritability of living tissues, and without such study 

 that investigation must in future be regarded as incomplete. 



In thus reviewing the movements of plants as a whole, in 

 the light of the investigations which have been described in 

 the course of the present work, we cannot have failed to be 

 struck by the fact that all alike represent, under changing 

 conditions, a single fundamental responsive phenomenon ; 

 and this response of the plant offers us, as we have seen, 

 a means of tracing the process by which physiological 

 differentiation from simple to complex has taken place, under 

 the action of stimulus itself. It also enables us to refer a 

 specific differentiation back to definite forces, which have 

 acted asymmetrically upon the organism and induced such a 

 change. We are thus able to see that responsive movements 

 apparently opposite in kind, are nevertheless traceable, not 

 to different specific sensibilities, but to a single universal 

 sensibility, finding different expressions by reason of these 

 induced physiological differentiations. 



Theory of Darwin. — It will be remembered that, accord- 

 ing to the theory of Darwin, a given individual variation 

 which might be in any way advantageous to the organism 

 was perpetuated by the process of natural selection. Thus, in 

 the struggle for existence, only those could continue to live 



