36 CHILDHOOD OF ANIMALS 



larvae are modified in many ways to suit the needs of their own life, 

 and it is a difficult judgment to decide in any case how much of 

 the character of a larva is adaptive and how much ancestral. In 

 both groups the continuous series may be interrupted at any point, 

 by the obliteration or telescoping of some of the stages, with the 

 result that occasionally a moult is preceded by a resting phase in 

 which the larva is more or less torpid and motionless, and when the 

 form that emerges from the moult is widely different from the 

 preceding form. Such bigger jumps give rise to the familiar 

 metamorphoses, and they are most frequent and most decided when 

 they are associated with a change in the habits of the creatures 

 before and after the metamorphic moult. 



There is one striking difference between the two groups. Amongst 

 insects the campodeiform larva, which is certainly the most primi- 

 tive, represents the most primitive group of living insects, and, more- 

 over, helps to link insects with another group, the group of centipede- 

 like animals. In Crustacea, the nauplius larva, which is certainly 

 the most primitive, does not represent any living group of Crus- 

 tacea and does not link the Crustacea directly with any other group. 

 Unlike the campodea larva which, but for the absence of reproductive 

 organs, has the appearance and characters of an adult animal, the 

 nauplius larva is plainly an immature creature. 



When Darwin first convinced naturalists that the living world as 

 we see it now had come into existence by a process of evolution, the 

 resemblance amongst the larvae of different animals, the resemblances 

 of the larvae of one set of animals with the adults of lower animals, 

 and the parallel between the larval development and the possible 

 ancestral history were thought to provide almost clear proof of the 

 fact of evolution and to show the actual path of evolution. But 

 although increase of knowledge has strengthened the general case 

 for evolution to such an extent that a reasonable naturalist can no 

 longer doubt it, we are getting more wary as to particular cases. 

 The struggle for existence amongst larvae is extremely severe ; of 

 the multitudes that are hatched, only a few reach adult life, and 

 then only after having escaped almost incredible dangers. And so 

 larvae have been shaped and moulded, coloured and armed in a 

 multitude of ways that fit them to the conditions in which they 

 live. And in this process they must have lost much of their in- 

 herited ancestral characters and must have acquired many delusive 

 resemblances. 



