teriorated in structure, yet
some authors[322] believe that they are eminently liable to malformations;
and this is not improbable, as everything which lessens the vital powers
acts in this manner. Instances of this kind have been recorded in the case
of pigs, bloodhounds, and some other animals.

Finally, when we consider the various facts now given which plainly show
that good follows from crossing, and less plainly {144} that evil follows
from close interbreeding, and when we bear in mind that throughout the
whole organic world elaborate provision has been made for the occasional
union of distinct individuals, the existence of a great law of nature is,
if not proved, at least rendered in the highest degree probable; namely,
that the crossing of animals and plants which are not closely related to
beach other is highly beneficial or even necessary, and that interbreeding
prolonged during many generations is highly injurious.

* * * * *

{145}

CHAPTER XVIII.

ON THE ADVANTAGES AND DISADVANTAGES OF CHANGED CONDITIONS OF LIFE:
STERILITY FROM VARIOUS CAUSES.

ON THE GOOD DERIVED FROM SLIGHT CHANGES IN THE CONDITIONS OF
LIFE--STERILITY FROM CHANGED CONDITIONS, IN ANIMALS, IN THEIR NATIVE
COUNTRY AND IN MENAGERIES--MAMMALS, BIRDS, AND INSECTS--LOSS OF
SECONDARY SEXUAL CHARACTERS AND OF INSTINCTS--CAUSES OF
STERILITY--STERILITY OF DOMESTICATED ANIMALS FROM CHANGED
CONDITIONS--SEXUAL INCOMPATIBILITY OF INDIVIDUAL ANIMALS--STERILITY OF
PLANTS FROM CHANGED CONDITIONS OF LIFE--CONTABESCENCE OF THE
ANTHERS--MONSTROSITIES AS A CAUSE OF STERILITY--DOUBLE
FLOWERS--SEEDLESS FRUIT--STERILITY FROM THE EXCESSIVE DEVELOPMENT OF
THE ORGANS OF VEGETATION--FROM LONG-CONTINUED PROPAGATION BY
BUDS--INCIPIENT STERILITY THE PRIMARY CAUSE OF DOUBLE FLOWERS AND
SEEDLESS FRUIT.

_On the Good derived from slight Changes in the Conditions of Life._--In
considering whether any facts were known which might throw light on the
conclusion arrived at in the last chapter, namely, that benefits ensue from
crossing, and that it is a law of nature that all organic beings should
occasionally cross, it appeared to me probable that the good derived from
slight changes in the conditions of life, from being an analogous
phenomenon, might serve this purpose. No two individuals, and still less no
two varieties, are absolutely alike in constitution and structure; and when
the ge