230 CHILDHOOD OF ANIMALS 



rience, and there remains a strong instinctive side in intelligence. 

 The modifications of instinct may last only a very short time or 

 they may remain almost permanent. They may be not much 

 more than purely mechanical changes, as when a new pen after a 

 little use writes more smoothly, or the other kind of change that 

 is known as fatigue, as when a watch-spring, repeatedly used, 

 must be allowed a rest to recover its normal reaction. From these 

 simple changes the effects of experience on instinct grade up towards 

 memory, and at least in human beings into conscious memory. 

 The period of youth is the time when instinct is gradually broken 

 down and replaced by experimental action. 



Let us get the matter clear by some examples of undoubted 

 instinct. A caterpUlar is developed from an egg laid on a leaf. 

 It has never seen its mother. It has never done anything all 

 through its life, except eat when it was hungry (and that was most 

 of the time), crawl under the leaf when it rained, come out again 

 and resume eating when it was dry, and, perhaps, when it was 

 startled, drop suddenly down, spinning a thread of silk, by which 

 it re-ascended after a time. Suddenly, and once only in its lifetime, 

 it completely changes its habits. It spins silk without having 

 been disturbed, roUs itself up in a leaf and fastens the edges of 

 this blanket with threads of silk. All the caterpillars in the same 

 brood do this exactly in the same way and almost exactly at the 

 same time. They either accomplish their task correctly or bungle 

 it completely. There is no question of practice, or imitation, or 

 of learning. The necessary act is accomplished once and for all. 

 When I was at Oxford, I used to keep common garden spiders 

 in a cage, and found that when they were provided with twigs and 

 proper surfaces to which they could attach the anchoring spokes 

 of their webs, they spun these always exactly in the same way, and 

 that in watching them the action seemed so orderly and was so 

 completely fitted for its object that it was difficult not to think 

 of it as intelligent. But one of the spiders, placed under an inverted 

 bell-jar to which the threads would not remain fastened, quite 

 contentedly went through the exact routine of operations for making 

 a web, although the result was a meaningless wisp of threads. A 

 chick that has been blinded by covering its eyes with a hood almost 

 before it has got out of the shell, and that is kept blindfolded and 

 fed by hand for a day or two until it is strong, will peck at objects 

 unerringly as soon as it is allowed to see them, although it will not 

 at once distinguish between food and stones. My caracal cub 



