Thus we see that when organic beings are placed under new and
unnatural conditions, and when hybrids are produced by the unnatural
crossing of two species, the reproductive system, independently of the
general state of health, is affected by sterility in a very similar
manner. In the one case, the conditions of life have been disturbed,
though often in so slight a degree as to be inappreciable by us; in
the other case, or that of hybrids, the external conditions have
remained the same, but the organisation has been disturbed by two
different structures and constitutions having been blended into one.
For it is scarcely possible that two organisations should be
compounded into one, without some disturbance occurring in the
development, or periodical action, or mutual relation of the different
parts and organs one to another, or to the conditions of life. When
hybrids are able to breed inter se, they transmit to their offspring
from generation to generation the same compounded organisation, and
hence we need not be surprised that their sterility, though in some
degree variable, rarely diminishes.
It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent.
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