is not invariably brought into action, as in the
cases of parthenogenetic reproduction. We are here naturally led to inquire
what the final cause can be of the necessity in ordinary generation for the
concourse of the two sexual elements.

Seeds and ova are often highly serviceable as the means of disseminating
plants and animals, and of preserving them during one or more seasons in a
dormant state; but unimpregnated seeds or ova, and detached buds, would be
equally serviceable for both purposes. We can, however, indicate two
important advantages gained by the concourse of the two sexes, or rather of
two individuals belonging to opposite sexes; for, as I have shown in a
former chapter, the structure of every organism appears to be especially
adapted for the concurrence, at least occasionally, of two individuals. In
nearly the same manner as it is admitted by naturalists that hybridism,
from inducing sterility, is of service in keeping the forms of life
distinct and fitted for their proper places; so, when species are rendered
highly variable by changed conditions of life, the free intercrossing of
the varying individuals will tend to keep each form fitted for its proper
place in nature; and crossing can be effected only by sexual generation,
but whether the end thus gained is of sufficient importance to account for
the first origin of sexual intercourse is very doubtful. Secondly, I have
shown, from the consideration of a large body of facts, that, as a slight
change in the conditions of life is beneficial to each creature, so, in an
analogous manner, is the change effected in the germ by sexual union with a
distinct individual; and I have been led, from observing the many
widely-extended provisions throughout nature for this purpose, and from the
greater vigour of crossed organisms of all kinds, as proved by direct
experiments, as well as from the evil effects of close interbreeding when
long {363} continued, to believe that the advantage thus gained is very
great. Besides these two important ends, there may, of course, be others,
as yet unknown to us, gained by the concourse of the two sexes.

Why the germ, which before impregnation undergoes a certain amount of
development, ceases to progress and perishes, unless it be acted on by the
male element; and why conversely the male element, which is enabled to keep
alive for even four or five years within the spermatheca of a female
insect, likewise perishes, unless it act