rids._--I shall now conclude
this subject by summing up, in a few words, the results to which I have
been led by the consideration of the phenomena of hybrids. It appears
that the aversion of individuals of distinct species to the sexual union
is common to animals and plants; and that it is only when the species
approach near to each other in their organization and habits, that any
offspring are produced from their connexion. Mules are of extremely rare
occurrence in a state of nature, and no examples are yet known of their
having procreated in a wild state. But it has been proved, that hybrids
are not universally sterile, provided the parent stocks have a near
affinity to each other, although the continuation of the mixed race, for
several generations, appears hitherto to have been obtained only by
crossing the hybrids with individuals of pure species; an experiment
which by no means bears out the hypothesis that a true hybrid race could
ever be permanently established.

Hence we may infer, that aversion to sexual intercourse is, in general,
a good test of the distinctness of original stocks, or of _species_; and
the procreation of hybrids is a proof of the near affinity of species.
Perhaps, hereafter, the number of generations for which hybrids may be
continued, before the race dies out (for it seems usually to degenerate
rapidly), may afford the zoologist and botanist an experimental test of
the difference in the degree of affinity of allied species.

I may also remark, that if it could have been shown that a single
permanent species had ever been produced by hybridity (of which there is
no satisfactory proof), it might certainly have lent some countenance to
the notions of the ancients respecting the gradual deterioration of
created things, but none whatever to Lamarck's theory of their
progressive perfectibility, for observations have hitherto shown that
there is a tendency in mule animals and plants to degenerate in
organization.

It was before remarked, that the theory of progressive development arose
partly from an attempt to ingraft the doctrines of the transmutationists
upon one of the most popular generalizations in geology. But we have
seen in the ninth chapter, that the modern researches of geologists have
broken at many points the chain of evidence once supposed to exist in
favor of the doctrine, that, at each successive period in the earth's
history, animals and plants of a higher grade, or more complex