52 MODIFICATION IN THE MOUTH. 



in the constitution of the mouth, although in the 

 majority of cases the result is also accompanied by 

 changes in the legs and in the internal organization. 



It is, however, obvious, that a mouth like that of a 

 beetle could not be modified into a suctorial organ like 

 that of a bug or a gnat, because the intermediate stages 

 would necessarily be injurious. Neither, on the other 

 hand, could the mouth of the Hemiptera be modified, 

 for the same reason, into a mandibulate type like that 

 of the Coleoptera. But in Gamjwclea and the Collem- 

 BOLA we have a type of animal closely resembling 

 certain larvse which occur both in the mandi- 

 bulate and suctorial series of insects, in Lepid- 

 optera, Hymenoptera, Orthoptera, and Coleoptera, 

 and which possesses a mouth neither distinctly 

 mandibulate nor distinctly suctorial, but constituted on 

 a peculiar type, capable of modification in either direc- 

 tion by gradual changes without loss of utility. 



These considerations seem to me strongly to sup- 

 port M. Brauer's conclusion, that in the very interest- 

 ing genus Gampodea we have a form closely resembling 

 the parent stock, from which the various orders of 

 insects have arisen ; and at the same time they seem 

 to me to throw some light on the opposite mouth- 

 types which occur in the same species throughout so 

 many groups of insects, a fact which has long seemed 

 to me one of the greatest difficulties in the theory of 

 natural selection. 



Yet I cannot agree with Brauer and MiiUer in re- 

 garding the type represented by the " grub " or " mag- 

 got " as altogether of more recent origin than the 

 Gampodea form. The sequence in the evolution of 

 insects seems to me to be as follows : — First, an Apod, 

 fleshy, vermiform, maggot-like being ; of which there 

 are two types, that of the fly- maggot, and the more 

 highly organized bee-grub. 



Secondly, the Gampodea stage, from which the various 

 orders of insects have diverged in different directions, 

 and to a greater or less extent. The Gampodea itself 



