﻿NORTH AMERICAN SPHINGID^]. 109 



respect, they are subject to marked variations in the individuals of the best character- 

 ized genera of the group. In consequence of this conviction, I have not described 

 this portion of the structure of the perfect insect in my own diagnoses, preferring 

 rather to assign to it another and truer value. At the same time, I am well aware, 

 that in other groups, in common with all parts of structure, it undergoes changes 

 which are significant and valuable guides to the student. In the papers which may 

 succeed the present one, these variations will be duly delineated and described, as they 

 may occur in the various families. But, the valuation which will be presently assigned 

 to this structural part, is intended to be a general one, which will apply to all families 

 of the division Heterocera. 



I hope the student will find the treatment of this group sufficiently lucid to give 

 him a clear conception of it. It was not possible to confine its consideration to a 

 single group, or to avoid the introduction of the individual, but this I trust has not 

 given rise to want of perspicuity. I will candidly acknowledge, that I have found the 

 communication of my own understanding of the nature of the group, a subject of no 

 slight difficulty, and whether I have been successful in the endeavor, the student must 

 determine for himself. — The recognition of generic differences in the perfect being, is 

 usually regarded as a happy endowment, or matter of individual tact, especially dis- 

 tinguishing a few fortunately constituted minds. I should rather characterize it as the 

 consequence of the observance of fixed principles founded in nature, and the habit of 

 duly valuing differences in ultimate structure. We will conclude the consideration of 

 the group with the attempt to define the Genus as, a group of specific forms, 

 deviating from all others of the same family, or an expression of conformity in the specific 

 cycles of distinct species, and indicated in the individual by the peculiarities of the ulti- 

 mate structure of some of its parts. 



f A M I L Y. 



During the process of classification it is found, that assemblages of generic groups 

 naturally form a third and more extensive one. For, as has been observed heretofore, 

 the limits of genera are not abrupt usually, but shade into each other, retaining those 

 characteristics of form which result from internal organization and structure, but de- 

 viating in the ultimate structure of some of their parts. Form, indeed, is the expres- 

 sion of structure, and though modified in its details, the fundamental idea of which 

 the genus is a special group, is retained so as to be recognizable, and we perceive in 

 the individuals of various genera a resemblance to each other, not only in shape, but 

 in those parts of structure which go to determine shape. And this is not only ob- 

 servable in the perfect state, but is perhaps most significantly shown in the embryo, 

 in consequence of the simplicity of their figures. In the perfect insect, this idea of 



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