66 ON THE ORIGIN AND [CHAP. 



species the development of the egg presents certain 

 differences. 1 



The metamorphoses of insects depend then pri- 

 marily on the fact that the young quit the egg at 

 a more or less early stage of development ; and 

 that consequently the external forces, acting upon 

 them in this state, are very different from those by 

 which they are affected when they arrive at maturity. 



Hence it follows that, while in many instances 

 mature forms, differing greatly from one another, 

 arise from very similar larvae, in other cases, as we 

 have seen, among some the parasitic Hymenoptera, 

 insects agreeing closely with one another, are pro- 

 duced from larvae which are very unlike. The same 

 phenomenon occurs in other groups. Thus, while in 

 many cases very dissimilar jelly-fishes arise from 

 almost identical Hydroids, we have also the reverse 

 of the proposition in the fact that in some species, 

 Hydroids of an entirely distinct character produce 

 very similar Medusae.. 2 



We may now pass to the second part of our sub- 

 ject: the apparent suddenness and abruptness of 

 the changes which insects undergo during metamor- 

 phosis. But before doing so I must repeat that these 

 changes are not always, even apparently, sudden and 

 great. The development of an Orthopterous insect, 

 say a grasshopper, from its leaving T the egg to 

 maturity, is so gradual that the ordinary nomen- 



1 For differences in larva consequent on variation in the external con- 

 dition, see ante, p. 61. 



2 See Hincks. British Hydroid Zoophytes, P. Ixii. Agassiz, Sea-side 

 Studies, p, 43. 



