One of the objects of zoological study is to ascertain more completely and more accurately the peculiarities by which one kind of animal (species, variety, genus, &c.) is distinguished from another. The advance of our knowledge in this direction depends on the more minute examination and more accurate distinction of known forms, the examination of larger numbers of specimens from familiar localities, and the examination of specimens from localities previously unsearched. There is scarcely any family so thoroughly investigated that it does not yield new discoveries on a renewed examination of more abundant material. It is found possible to recognise finer distinctions, and so split up one species into several, or convert what was considered a species into a genus. New material—that is to say, examination of a large number of specimens— often shows, too, that distinct species are more or less connected by intermediate forms. But in all this work the part played by these minute peculiarities in the life of the animal usually receives little attention. It is not the object of systematic zoology to ascertain the uses of characters, or to explain their origin. These objects require different methods, and are usually pursued by different investigators. But among the various methods employed there is one which has seldom, if ever, been followed—that of surveying the various characters of different grades,—specific, generic, family, &c.—in order to find whether it is possible to trace a connection between them and the habits of the animals which exhibit them, and generally to consider how far the principles which have been suggested in explanation of the evolution of species are applicable to the diagnostic characters of a particular family.