240 DEVELOPMENT AND EMBEYOLOGY. [Chap. XIY. 



finally are developed into mature males and females, 

 propagating their kind in the ordinary manner by eggs. 



It may be worth notice that when Wagner's remark- 

 able discovery was first announced, I was asked how 

 was it possible to account for the larvae of this fly 

 having acquired the power of asexual reproduction. As 

 long as the case remained "unique no answer could be 

 given. But already Grimm has shown that another fly, 

 a Chironomus, reproduces itself in nearly the same 

 manner, and he believes that this occurs frequently in 

 the Order. It is the pupa, and not the larva, of the 

 Chironomus which has this power ; and Grimm further 

 shows that this case, to a certain extent, " unites that 

 of the Cecidomyia with the parthenogenesis of the 

 Coccidee;" — the term parthenogenesis implying that 

 the mature females of the Coceidse are capable of 

 producing fertile eggs without the concourse of the 

 male. Certain animals belonging to several classes are 

 now known to have the power of ordinary reproduction 

 at an unusually early age ; and we have only to acceler- 

 ate parthenogenetic reproduction by gradual steps to an 

 earlier and earlier age, — Chironomus showing us an 

 almost exactly intermediate stage, viz., that of the pupa 

 — and we can perhaps account for the marvellous case 

 of the Cecidomyia. 



It has already been stated that various parts in 

 the same individual which are exactly alike dur- 

 ing an early embryonic period, become widely dif- 

 ferent and serve for widely different purposes in 

 the adult state. So again it has been shown that 

 generally the embryos of the most distinct species 

 belonging to the same class are closely similar, but 

 become, when fully developed, widely dissimilar. A 



