THE PURPOSE OF YOUTH 235 



between bodies of matter that are not alive, for the living matter 

 is constantly changing and almost from moment to moment may 

 behave differently with regard to the same stimulus. They do 

 not occur separately and independently, but in all the higher forms 

 they are combined in varying degrees. But without doubt they 

 are, so to say, the raw materials or uncombined elements of the 

 instincts. A new-born mammal pressing against its mother and 

 seeking the nipple acts through a complicated nervous and muscular 

 machinery so nicely adapted to its purpose that it can hardly fail 

 to act. But it acts through various tropisms, the reactions to 

 warmth, to contact, and to chemical stimulation through taste 

 and smell. 



These various factors of instinct can be modified by experience. 

 In a few cases the same response is repeated to each application of 

 the stimulus, but it is far more usual for a change in the response 

 to take place, the duration of the change increasing in the higher 

 animals until it passes into what can be called memory. An in- 

 fusorian animalcule will go on bumping up against an obstacle 

 indefinitely, at each contact recoiling, twisting over and charging 

 again, and it is mere luck if in a series of movements of this kind 

 it finally discovers a way round. A worm similarly confronted 

 with an obstacle behaves practically in the same way, but it gets 

 more and more excited in its movements, and may finally get round 

 by some violent contortion, more or less in the fashion that a man 

 reading a book will, from time to time, simply put up his hand to 

 push away a persistent blue-bottle, but at last will get out of his 

 chair and hunt it round the room. In the latter case, however, 

 in addition to the summation of the irritating effect of the stimulus, 

 the higher parts of the man's brain come into play, and he ceases 

 to be merely insti ctive. A fish which was kept in a tank was 

 accustomed to come to a particular place for a piece of food that 

 was dropped in. Then a sheet of glass was fixed between the 

 habitual lurking-place and the spot where the food was dropped. 

 For some time the fish continued to dart out on the food, bumped 

 up against the glass, retired and returned to the charge. Presently, 

 however, it ceased to respond, and, for a few days afterwards, no 

 longer rushed at the food even although the glass had been removed. 

 It is the natural instinct of a spider to drop from its web at any 

 sudden vibration, or when a shadow is thrown on it. Mr. and 

 Mrs. Peckham, experimenting with spiders, found that they would 

 drop when a tuning-fork was sounded near them, but that after 



