ANIMAL AND VEGETABLE 181 



same way the auditory nerve is impelled to the perception 

 of sound, not merely by waves of sound, but by every 

 change which affects it, and similarly with the remaining 

 organs of sense. 



" Now I pointed out years ago that even the organs of 

 plants are provided with similar specific energies. Irritable 

 organs in plants are, indeed, like' the sense-organs of 

 animals, sensitive to a definite category of stimuli, but 

 they can very often be affected by other stimuli also, and 

 in this case the stimulation is always the same. This 

 appears most distinctly, for example, in the case of growing 

 internodes and leaves. If they are illuminated from one 

 side they become curved, and if brought out of their 

 normal position they are caused to make exactly similar 

 curvatures : the one mode possible for responding to any 

 stimulus whatever is simply this curving. The matter 

 only obtains its full significance by the fact that every 

 individual plant-organ responds to the influence of light as 

 well as to that of gravitation in a manner specifically 

 peculiar to it, and it is upon this that the anistropy of the 

 parts of plants depends. No less clear is the specific 

 energy of tendrils. . . . The identity of the effect of 

 stimulation in cases where totally different stimuli act on 

 the growing root-tips is particularly striking. . . . The 

 organ possesses only one mode of responding to stimuli of 

 the most various kinds. . . . The organism itself is only 

 the machine, consisting of various parts, and which must 

 be set in motion by the action of external forces : it de- 

 pends upon its structure what effects these external forces 

 produce in it. 



" It would betray a very low level of scientific culture 

 to see in this comparison a degradation of the organism, 

 since in a machine, although only constructed by human 

 hands, there lies the result of the most profound and care- 

 ful thought and high intelligence, so far as its structure is 



