no other category of structure as characteristic of Genera than the details of structure by which members of the same Family may differ from each other, and this I consider as the only true basis on which to limit Genera, while it is at the same time in perfect accordance with the practice of the most eminent modern zoologists.  It is in this way that Cuvier has distinguished the large number of Genera he has characterized in his great Natural History of the Fishes, in connection with Valenciennes.  Latreille has done the same for the Crustacea and Insects; and Milne Edwards, with the cooeperation of Haime, has recently proceeded upon the same principle in characterizing a great number of Genera among the Corals.  Many others have followed this example, but few have kept in view the necessity of a uniform mode of proceeding, or, if they have done their researches have covered too limited a ground, to be taken into consideration in a discussion of principles.  It is, in fact, only when extending over a whole Class that the study of Genera acquires a truly scientific importance, as it then shows in a connected manner, in what way, by what features, and to what extent a large number of animals are closely linked together in Nature.  Considering the Animal Kingdom as a single complete work of one Creative Intellect, consistent throughout, such keen analysis and close criticism of all its parts have the same kind of interest, in a higher degree, as that which attaches to other studies undertaken in the spirit of careful comparative research.  These different categories of characters are, as it were, different peculiarities of style in the author, different modes of treating the same material, new combinations of evidence bearing on the same general principles.  The study of Genera is a department of Natural History which thus far has received too little attention even at the hands of our best zoologists, and has been treated in the most arbitrary manner; it should henceforth be made a philosophical investigation into the closer affinities which naturally bind in minor groups all the representatives of a natural Family.

Genera, then, are groups of a more restricted character than any of those we have examined thus far.  Some of them include only one Species, while others comprise hundreds; since certain definite combinations of characters may be limited to a single Species, while other combinations may be repeated in many.  We have striking examples of this among Birds:  the Ostrich stands alone in its Genus, while the number of Species among the Warblers is very great.  Among Mammalia the Giraffe also stands alone, while Mice and Squirrels include many Species.  Genera are founded, not, as we have seen, on general structural characters, but on the finish of special parts, as, for instance, on the dentition.  The Cats have only four grinders in the upper jaw and three in the lower, while the Hyenas have one more above and below, and the Dogs and Wolves have two more above and two