ss, _Equus asinus_; and
the zebra, _Equus zebra_. By a further extension of this principle of
classification, very closely allied genera are united under the term
of _family_.

The different varieties of the same species breed, as might be
anticipated, freely together; but it frequently happens that two
individuals of different species pair, and produce an animal which
inherits some of the properties of each of its progenitors. These
half-breeds are termed _hybrids_, or _mules_, and we have familiar
examples of them in the common mule and the jennet. As a general rule,
animals exhibit a disinclination to breed with other than members of
their own species; and although the interference of man may overcome
this natural repugnance, he can only effect the fruitful congress of
individuals belonging to closely allied species, being members of the
same genus. Hybrids in the genus _Equus_ are very common. A cross has
been produced between the he-goat and the ewe; the camel and the
dromedary have bred together; and Buffon succeeded in producing a hybrid
in which three animals were represented--namely, the bison, the zebu,
and the ox. On the other hand, attempts to effect a cross between
animals belonging to different families have generally failed; nor is
it at all probable that a cross will ever be produced between the pig
and the sheep, between the horse and the cow, or, most unlikely of all,
between the dog and the cat.

It is the general belief that hybrids are sterile, or, at least, that
they are incapable of propagation _inter se_. This may be true with
respect to the hybrids of species not very closely allied; but that
there are exceptions to the rule is quite clear from Roux's experiments
with hares and rabbits. This gentleman, who is, or was, the president
of a French agricultural society, but who makes no profession of
scientific knowledge, has succeeded, after several failures, in producing
a fruitful cross between the rabbit and the hare. This hybrid has
received the name of leporide (from the Latin _leporinus_, pertaining to
a hare), and it is different from former crosses, in being five parts
hare, and three parts rabbit. M. Roux has bred this hybrid during the
last eighteen years, and has not observed the slightest appearance
of decay of race manifest itself up to the present, so that, for all
practical purposes, the leporide may be regarded as an addition to the
distinct species of animals. The leporide fattens