more below.  In the last, some of the teeth have also flat surfaces for crushing the food, adapted especially to their habits, since they live on vegetable as well as animal substances.  The formation of the claws is another generic feature.  There is a curious example with reference to this in the Cheetah, which is again a Genus containing only one Species.  It belongs to the Cat Family, but differs from ordinary Lions and Tigers in having its claws so constructed that it cannot draw them back under the paws, though in every other respect they are like the claws of all the Cats.  But while it has the Cat-like claw, its paws are like those of the Dog, and this singular combination of features is in direct relation to its habits, for it does not lie in wait and spring upon its prey like the Cat, but hunts it like the Dog.

While Genera themselves are, like Families, easily distinguished, the characters on which they are founded, like those of Families, are difficult to trace.  There are often features belonging to these groups which attract the attention and suggest their association, though they are not those which may be truly considered generic characters.  It is easy to distinguish the Genus Fox, for instance, by its bushy tail, and yet that is no true generic character; the collar of feathers round the neck of the Vultures leads us at once to separate them from the Eagles, but it is not the collar that truly marks the Genus, but rather the peculiar structure of the feathers which form it.  No Bird has a more striking plumage than the Peacock, but it is not the appearance merely of its crest and spreading fan that constitutes a Genus, but the peculiar structure of the feathers.  Thousands of examples might be quoted to show how easily Genera may be singled out, named, and entered in our systems, without being duly characterized, and it is much to be lamented that there is no possibility of checking the loose work of this kind with which the annals of our science are daily flooded.

It would, of course, be quite inappropriate to present here any general revision of these groups; but I may present a few instances to illustrate the principle of their classification, and to show on what characters they are properly based.  Among Reptiles, we find, for instance, that the Genera of our fresh-water Turtles differ from each other in the cut of their bill, in the arrangement of their scales, in the form of their claws, etc.  Among Fishes, the different Genera included under the Family of Perches are distinguished by the arrangement of their teeth, by the serratures of their gill-covers, and of the arch to which the pectoral fins are attached, by the nature and combination of the rays of their fins, by the structure of their scales, etc.  Among Insects, the various Genera of the Butterflies differ in the combination of the little rods which sustain their wings, in the form and structure of their antennae, of their feet,