hem as
countenancing the opposite doctrine, that all the varieties of
organization and instinct now exhibited in the animal and vegetable
kingdoms may have been propagated from a small number of original types.

In regard to the mammifers and birds it is found that no sexual union
will take place between races which are remote from each other in their
habits and organization; and it is only in species that are very nearly
allied that such unions produce offspring. It may be laid down as a
general rule, admitting of very few exceptions among quadrupeds, that
the hybrid progeny is sterile; and there seem to be no well
authenticated examples of the continuance of the mule race beyond one
generation. The principal number of observations and experiments relate
to the mixed offspring of the horse and the ass; and in this case it is
well established that the he-mule can generate, and the she-mule
produce. Such cases occur in Spain and Italy, and much more frequently
in the West Indies and New Holland; but these mules have never bred in
cold climates, seldom in warm regions, and still more rarely in
temperate countries.

The hybrid offspring of the she-ass and the stallion, the [Greek:
ginnos] of Aristotle, and the hinnus of Pliny, differs from the mule, or
the offspring of the ass and mare. In both cases, says Buffon, these
animals retain more of the dam than of the sire, not only in the
magnitude, but in the figure of the body: whereas, in the form of the
head, limbs, and tail, they bear a greater resemblance to the sire. The
same naturalist infers, from various experiments respecting cross-breeds
between the he-goat and ewe, the dog and she-wolf, the goldfinch and
canary-bird, that the male transmits his sex to the greatest number, and
that the preponderance of males over females exceeds that which prevails
where the parents are of the same species.

_Hunter's opinion._--The celebrated John Hunter has observed, that the
true distinction of species must ultimately be gathered from their
incapacity of propagating with each other, and producing offspring
capable of again continuing itself. He was unwilling, however, to admit
that the horse and the ass were of the same species, because some rare
instances had been adduced of the breeding of mules, although he
maintained that the wolf, the dog, and the jackal were all of one
species; because he had found, by two experiments, that the dog would
breed both with the wolf and the