CHAPTER LII 



ON PHYSIOLOGICAL RESPONSE, 

 AND ITS CONTINUITY IN PLANT AND ANIMAL 



Vitalism — Fundamental unity of physiological response in plant and animal — 

 Theory of Darwin — Variation as induced by external forces. 



We have reviewed, in the last two chapters, the various 

 phenomena of plant response. We shall now turn our atten- 

 tion to the consideration of irritability,* or the capacity of 

 responding to stimulus, in general. 



Vitalism. — We have seen that when a tissue is rendered 

 molecularly sluggish by any physical means, such as cooling, 

 its irritability is found to be temporarily abolished. Irritability 

 is thus ultimately due to molecular responsiveness, and ex- 

 citatory response is brought about by the molecular derange- 

 ment consequent on stimulus, with the subsequent self- 

 recovery. We have seen further that the state of excitation 

 is exhibited, either by a mechanical or by the electrical mode 

 of response, and that even where mechanical indications are 

 not available, the electrical sign of excitation is unfailing. 

 We have also seen briefly in the course of the present work, 

 and I have demonstrated in full elsewhere, 1 the fact that 

 similar excitatory response is given, even by inorganic matter, 

 under stimulation ; and in such cases also we have been able 

 to observe and record not only the phenomenon of response 

 itself, but also its numerous appropriate modifications under 

 varying conditions. Thus fatigue brings about diminution 

 of inorganic, as of organic response. Amongst chemical 

 reagents, again, some induce exaltation and others depres- 

 sion ; and many so-called poisons act here, as in the case 



1 Bose, Response in the Living and Non- Living, 1902. 



