(162.) The characters by which natural groups, like those we have hitherto contemplated, are to be known and designated, has been a fruitful subject of disquisition among writers. It has been customary, until within the last few years, for naturalists to decide, à priori, upon those characters which a group of species, or a single one, should possess, in order to constitute a genus. This mode of proceeding, as may naturally be supposed, led every one to follow his own opinion; so that almost every part of an animal, in turn, had been singled out as the most important for this purpose. Thus, Linnæus founded his genera of birds entirely on the form of the bill and the construction of the feet; totally disregarding the formation of their wings, — which is one of the chief characteristics of birds, — and entirely overlooking their manners, habits, and food. In entomology, however, he constructed his genera on a totally different principle. Here he considers the wings of insects as affording the most important characters; and he has accordingly founded all his great divisions, and most of his lesser ones, in the different modifications which these members present; while the mouth and the feet, which were so highly regarded in his arrangement of birds, are scarcely noticed in his classification of insects. Fabricius, on the other hand, as if determined to fly to the other extreme, takes all his leading characters from those parts of insects which his great master regarded as insignificant.