The next grade in ascent from the species is invariably the GENUS, for subgenera, like varieties in species, are not uniformly present, but are mere contingencies, even if they do properly exist.
Why some genera abound in species and others are so limited is as difficult to determine as the differing numerical abundance of individuals in species. That long genera (genera numerous in species) may be the result of natural selection, as Mr. Darwin surmises, and the offspring of a common parentage, is contradicted, not merely by peculiar although sometimes slight dissimilarities of habit, combined with size and colour, but also if any lines of demarcation are to be admitted, it is possible, were their generic similitude to be subjected to severe test, they might present characteristics normally discrepant and suggestive of further division, although the habit may be very like.
The generic grouping is effected by structural peculiarities, which are essentially of a higher class than the characters of specific separation, these being determined by colour, pubescence, sculpture, etc. etc.; specific characters combining only individuals with such peculiar inferior resemblances. The generic characters thus establish groups of species allied only by such more general character and similarity, but conjunctively of one permanent habit, although the members of the genus may differ somewhat in habits, and so on of the higher groups into which insects are collected, each group in its ascent upwards presenting characteristics of a wider range than those of the descending series. And so, by degrees, we rise until we reach the characters which combine the whole order. The process is necessarily and imperatively synthetical, for the whole foundation is based upon species, and thence emanates the supposition that only species exist.
The _type_ of a genus is that species upon the characters of which the genus was originally framed and named, and theoretically, however generic groups may be subsequently divided to suit views or to meet systems, the primitive generic type is assumed to retain the primitive generic name. It is much to be doubted whether, in every case, the type is the true pattern, or leader, or centre of the group called the genus; nor is it likely if genera be natural groups. It has usually been accident which has dropped upon the favoured species, and not a well-calculated and thoroughly digested selection, and which, although accepted, will require emendation or change if the whole collective series should ever be obtained.