ural Genus, it is not difficult to ascertain what are essentially
specific characters. There is hardly among Mammalia a more natural Genus
than that which includes the Rabbits and Hares, or that to which the
Rats and Mice are referred. Let us see how the different Species differ
from one another. Though we give two names in the vernacular to
the Genus Hare, both Hares and Rabbits agree in all the structural
peculiarities which constitute a Genus; but the different Species are
distinguished by their absolute size when full-grown,--by the nature and
color of their fur,--by the size and form of the ear,--by the relative
length of their legs and tail,--by the more or less slender build of
their whole body,--by their habits, some living in open grounds,
others among the bushes, others in swamps, others burrowing under the
earth,--by the number of young they bring forth,--by their different
seasons of breeding,--and by still minor differences, such as the
permanent color of the hair throughout the year in some, while in others
it turns white in winter. The Rats and Mice differ in a similar way:
there being large and small Species,--some gray, some brown, others
rust-colored,--some with soft, others with coarse hair; they differ also
in the length of the tail, and in having it more or less covered with
hair,--in the cut of the ears, and their size,--in the length of
their limbs, which are slender and long in some, short and thick in
others,--in their various ways of living,--in the different substances
on which they feed,--and also in their distribution over the surface
of the earth, whether circumscribed within certain limited areas
or scattered over a wider range. What is now the nature of these
differences by which we distinguish Species? They are totally distinct
from any of the categories on which Genera, Families, Orders, Classes,
or Branches are founded, and may readily be reduced to a few heads. They
are differences in the proportion of the parts and in the absolute size
of the whole animal, in the color and general ornamentation of the
surface of the body, and in the relations of the individuals to one
another and to the world around. A farther analysis of other Genera
would show us that among Birds, Reptiles, Fishes, and, in fact,
throughout the Animal Kingdom, Species of well-defined natural Genera
differ in the same way. We are therefore justified in saying that the
category of characters on which Species are based i