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 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
singul