2d0 Professor L. Agassiz on the Principles of 



tural adaptation which is closely analogous to the earliest 

 condition of development of the insects : that the Crustacea 

 constitute a class intermediate between the worms and 

 nsects, and not superior to the insects, as some naturalists 

 would have them ; inasmuch as the highest combination of 

 their rings presents us with an arrangement similar to that 

 of the pupa of insects, in which the joints of the head and of 

 the chest are combined in an immovable shield, as in the pupa 

 of insects, and in which the joints of the abdomen alone re- 

 main movable, is also the case among the highest Crustacea. 

 The position of the insects as the highest class, can no longer 

 be denied, when we consider that in them the body is at last 

 divided into three distinct regions, — head, chest, and abdo- 

 men — and that the locomotive appendages, which, in the 

 lower classes, are so numerous and uniform along the whole 

 length of the body, are reduced to the region of the chest, 

 and assume there a particular development. 



Again, the tranformation of the respiratory organs is an 

 additional evidence in favour of such an arrangement, as 

 will be admitted from the fact that worms and Crustacea 

 have chiefly a bronchial respiration, while in insects it 

 becomes aerial, at least in their perfect condition. 



Once upon this tract, it was easy to follow out the minor 

 changes which these animals undergo during their final 

 transformation, and to derive from the knowledge of these 

 changes sufficient information to assign a definite position to 

 all the subordinate groups in each of these classes. Taking 

 the insects for instance, into special consideration, we ascer- 

 tain readily that chewing insects rank below the sucking 

 tribes, as their larvse are chewing worms, provided with 

 powerful jaws, even in the case of those which, like Lepidop- 

 tera, have the most perfectly developed sucking apparatus in 

 their mature condition. 



Again, an investigation of the changes which the wings 

 undergo in their formation, and the manner in which they 

 are unfolded, when the perfect insect is hatched, led to the 

 discovery that Coleopterous insects, far from ranking high, 

 must be considered as lowest among insects, inasmuch as 

 the upper larval wings of Lepidoptera are a sort of elytra. 



