WHAT DO YOUNG ANIMALS KNOW? 199 



ling at first with a crowd of older birds which have 

 done the journey before. In this way, it is pointed 

 out, a tradition of the route would be passed down 

 through indefinite generations of birds. There 

 would be no mystery about the matter. The only 

 difficulty about this explanation is that the young 

 birds do not always go with the old. In the case 

 of one of the greatest migratory species of all — 

 namely, our common cuckoo — the young birds 

 leave our shores many weeks later than the old 

 ones. The conditions of hfe are so difficult for the 

 cuckoo that the young have to remain in this way 

 to attain their full strength and growth. How do 

 the young find the way without any guidance or 

 assistance from birds that have made the journey 

 before ? The answer sometimes given is that the 

 young birds do not go at all, or at all events go only 

 a short distance. There can be no doubt that this 

 explanation is incorrect. Any one who has kept a 

 young cuckoo through the autumn and winter 

 months will feel convinced as to the immense 

 distances which the young birds must traverse in 

 flight during this period. For months during the 

 declining season of the year every muscle of the 

 young cuckoo's body will appear to be tense with 

 the uncontrollable instinct of .flight which seems to 

 overmaster it. Preyer described the action of 

 very young naked hermit-crabs, which at an early 

 age have to find deserted shells in which to shelter 

 themselves. Soon after leaving the egg they rush 

 with extraordinary animation for suitable shells 

 that are given to them in the water. They examine 

 the opening at the mouth, and take up their quarters 

 inside with remarkable alacrity. But if it chances 



