6 



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 transformation of the respiratory organs is an additional 

 evidence in favor of such an arrangement, as will be admitted from 

 the fact that Worms and Crustacea have chiefly a branchial respira- 

 tion, while in Insects it becomes aerial, in their perfect condition 

 at least. 



Once upon this track, 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 consider- 

 ation, we ascertain readily that chewing Insects rank below the sucking 

 tribes, as their larvae are chewing worms, provided with powerful 

 jaws, even in the case of those which, like Lepidoptera, 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, which, after being cast in the last moulting, are succeeded 

 by the more perfect membranous wing, which, in its turn, undergoes 

 such a development as to assign to those Lepidoptera which have 

 their wings folded backwards and enclosing the body a position 

 below those in which the wings spread sideways, and the highest 

 position to those which raise their wings upwards : so that these 

 investigations have settled even the relative position of the secondary 

 minor groups in each of these orders, and though, as yet, imperfectly 

 traced out, they have at least shown the principle upon which a 

 natural classification of these animals might be carried into the most 

 minute details, without ever leaving any point to our arbitrary deci- 

 sion. Similar results have already been arrived at in other classes ; 

 as, for instance, among Medusae, where naked-eyed Discophori, with 

 alternate generations, must be considered as the lowest type, recalling, 

 in one of their conditions, the appearances of the inferior class of 

 polypi, when the covered-eyed Discophori, with their strobiloid 

 generation, begins in its lowest state with a medusoid polyp. 



Similar facts are known among Echinoderms, in which, among Cri. 



