20 THE CLASSIFICATION OF ITStSECTS 



two little horn-like processes, communicating with the thoracic tracheae, which are, 

 no doubt, analogous to the two respiratory thoracic tubes of the larva of Culex. 



These facts seem to me important, not only as showing a greater analogy in the 

 metamorphoses of different insects than has been supposed to exist, but also as af- 

 fording greater facilities for the study of the transformation of the pupa into the 

 perfect insect than is afforded in other families, where the pupa itself is hard, and 

 prevents the investigator from tracing the final changes which the insect undergoes 

 within its mummy-like envelope while assuming its last forms. But upon this 

 point I am not, at present, prepared to offer further remarks, and would only call 

 the attention of entomologists to the facilities thus afforded for investigation. 



V. Eelative Position of the Classes of Articulata. 



It is a very strange circumstance, that, in the classification of articulated animals, 

 so little attention has been paid to the metamorphoses as a guide to lead us in our 

 investigations, that comparative anatomy alone has been appealed to for a decision 

 in this question, and that the facts brought forward upon anatomical evidence should 

 have led to an arrangement differing so much from that to which embryology 

 Avould lead. 



It will be remembered that all anatomical zoologists and all comparative anato- 

 mists without exception have considered the Crustacea as highest among Articulata, 

 on the ground of higher structure ; being probably influenced by the presence of a 

 heart and the extensive circulation which exists in Crustacea as compared with In- 

 sects in which the dorsal vessel faintly reminds us of a heart, and the circulation is 

 so peculiar as to have escaped notice until lately. This inference from anatomical 

 data has, probably, been made under the influence of an old view, from which it is 

 so difficult to divest ourselves, that animals should form a natural series, and may 

 be arranged in one progressive line according to the gradation of their structure. 

 The all-important distinction, introduced by Cuvier, of different types, of four 

 distinct plans of structure, has not yet sufficiently penetrated the spirit of those 

 who have followed in his steps. These four types are not universally acknowledged 

 as independent ; they are not constantly viewed as centres radiating in different 

 directions, as they ought to be. Their relative connection with each other is 

 more generally considered than their peculiarities as distinct types. For, if these 

 great divisions of the animal kingdom were fully understood as distinct types, it 

 would follow naturally in the mind of every one, that what might be a character of 

 superiority in one group might not be so in the other ; what might lead to natural 

 combinations in one department might mislead in the estimation of relations in 

 another ; and the final impression would be, that in each type there is a peculiar 

 ruling principle, which must be considered by itself; and that we are not allowed to 

 bring the four types into connection with each other, unless upon the most general 

 considerations, when investigating the foundation of animal life in its different 

 general tendencies, and comparing these tendencies with each other to find out their 

 common foundation as well as their difference. But as soon as we enter into any 



