12 THE FOUNDATIONS OF ZOOLOGY 



significance of natural signs, generally approves the responsive 

 actions of living things, although we find that these living things 

 are often misled by signs which we know to be illusions, which, 

 while similar in some respects to those to which the organic 

 mechanism is adjusted, signify something quite different from the 

 normal or customary course of events. 



As the nature of living things often leads to injurious or de- 

 structive actions, instinct is said to be blind or mechanical ; for while 

 no one can say whether the actions of the hermit-crab or those 

 of the blow-fly, or those of the human infant, are voluntary or not, 

 they are no more than the nature of these living things would 

 lead one to expect, and this is as true when they are beneficial 

 as it is when they mislead. 



If the adjustments between living things and the external 

 world were always beneficial, I do not see how the question 

 whether or not their actions are voluntary could present itself; 

 but the complexity of external nature is inexhaustible, and few 

 natural adjustments are beneficial under all circumstances, for even 

 a response to gravitation may mislead. • 



A growing plant needs the moisture and the soluble food 

 which it may find under ground, in course of nature, by follow- 

 ing the stimulus of gravity, and it also needs the sunlight and 

 the air which, in the normal or natural order of things, are to be 

 reached by upward growth. As the seed germinates, the radicle, 

 stimulated by gravity, grows downwards, while the plumule, which 

 does not differ essentially from the radicle in specific gravity, is 

 impelled by its nature to grow upwards under the same stimulus; 

 but each part grows by means of internal energy, and, while 

 gravity is the stimulus which throws it into action, it is not the 

 means by which the vital changes are brought about. 



The response is beneficial, and the stimulus seems as trust- 

 worthy as anything in nature ; yet the seeds often fall into places 

 where it misleads, and if a germinating seed be placed on the 

 edge of a horizontal wheel which turns slowly at a rate which 

 makes the centrifugal force somewhat greater than the weight of 

 the seed, the plumule grows towards and the radicle away from 

 the centre, although no seeds which act thus can grow up to 

 produce seeds in their turn. If plants think, a matter on which 



