70 ON THE ORIGIN AND [CHAP. 



others as, for instance, Raphidia are quiescent at 

 first, but at length acquire sufficient strength to walk, 

 though still enclosed within the pupa-skin : a power 

 dependent partly on the fact that this skin is very 

 thin. Others again as, for instance, dragon-flies are 

 not quiescent on assuming the so-called pupa state 

 for any longer time than at their other changes of 

 skin. The inactivity of the pupa is therefore not 

 a new condition peculiar to this stage, but a pro- 

 longation of the inaction which has accompanied 

 every previous change of skin. 



Nevertheless the metamorphoses of insects have 

 always seemed to me one of the greatest difficulties 

 of the Darwinian theory. In most cases, the develop- 

 ment of the individual reproduces to a certain extent 

 that of the race ; but the motionless, imbecile pupa 

 cannot represent a mature form. No one, so far as 

 I know, has yet attempted to explain, in accordance 

 with Mr. Darwin's views, a life-history in which the 

 mouth is first mandibulate and then suctorial, as, for 

 example, in a butterfly. A clue to the difficulty may, 

 I think, be found in the distinction between deve- 

 lopmental and adaptive changes ; to which I have 

 called attention in a previous chapter. The larva 

 of an insect is by no means a mere stage in the deve- 

 lopment of the perfect animal. On the contrary, 

 it is subject to the influence of natural selection, 

 and undergoes changes which have reference entirely 

 to its own requirements and condition. It is evi- 

 dent, then, that while the embryonic development 

 of an animal in the egg may be an epitome of its 

 specific history, this is by no means the case with 



