i6 CHILDHOOD OF ANIMALS 



extremely unlike their parents, but which would be carried away 

 by the currents in brooks and rivers. When the young crayfish is 

 hatched, it is a miniature crayfish which has only to grow and to make 

 a few trivial changes to reach the adult form. 



The young animals in the second group appear in the world in a 

 form that is more or less like that of their parents, and reach 

 maturity by increase in size and by a gradual assumption of 

 the full character of the adult. Incidentally they show various 

 structures and characters that are of benefit only in the period of 

 youth and that have probably been acquired for that purpose. In 

 their younger stages they often recall the structure and appearance 

 of the younger stages of their nearest relations, and probably also of 

 the ancestors common to them and to their nearest relations. But 

 these ancestral resemblances are vague and uncertain ; the young 

 animals do not wish to display to us their pedigrees, but to become 

 adults as quickly and as directly as possible. Although, however, 

 it appears to be certain that animals do repeat, to some extent, the 

 history of their race in their individual lives, and compress into a 

 few weeks or months the results of countless centuries of evolution, 

 we cannot expect the repetition to be very perfect. And I think 

 we are led to the curious conclusion that the more directly an animal 

 develops, and the earlier it shows traces of what it is going to 

 become, the less it shows of its ancestral history. The path of evolu- 

 tion which was slowly traced by the ancestors of the animals alive 

 to-day, has been long and tortuous, sometimes direct for a time, often 

 twisting sharply to one side or the other, sometimes, perhaps, even 

 bending backwards. So far as it is possible, animals avoid these 

 devious ways in their individual lives and press on straight to the 

 goal. In the animal kingdom as a whole, and in each of its divisions, 

 the higher types tend to develop most directly and to show least 

 of their ancestral history. 



Consideration of the third group of young animals, in which the 

 young stages differ much from the adult stages, requires a separate 

 chapter. 



