478 ORIGIN OF SPECIES 



DEVELOPMENT AND EMBRYOLOGY 



This is one of the most important subjects in the whole 

 round of natural history. The metamorphoses of insects, 

 with which every one is familiar, are generally effected ab- 

 ruptly by a few stages ; but the transformations are in reality 

 immerous and gradual, though concealed. A certain ephem- 

 erous insect (Chloeon) during its development, moults, as 

 shown by Sir J. Lubbock, above twenty times, and each 

 time undergoes a certain amount of change; and in this 

 case we see the act of metamorphosis performed in a pri- 

 mary and gradual manner. Many insects, and especially cer- 

 tain crustaceans, show us what wonderful changes of struc- 

 ture can be effected during development. Such changes, 

 however, reach their acme in the so-called alternate genera- 

 tions of some of the lower animals. It is, for instance, an 

 astonishing fact that a delicate branching coralline, studded 

 with polypi and attached to a submarine rock, should pro- 

 duce, first by budding and then by transverse division, a 

 host of huge floating jelly-fishes; and that these should pro- 

 duce eggs, from which are hatched swimming animalcules, 

 which attach themselves to rocks and become developed into 

 branching corallines; and so on in an endless cycle. The 

 belief in the essential identity of the process of alternate 

 generation and of ordinary metamorphosis has been greatly 

 strengthened by Wagner's discovery of the larva or maggot 

 of £ fly, namely the Cecidomyia, producing asexually other 

 larvae, and these others, which 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 remarkable 

 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 m 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 



