similarity. In both cases the sterility is independent of general
health, and is often accompanied by excess of size or great luxuriance.
In both cases, the sterility occurs in various degrees; in both, the
male element is the most liable to be affected; but sometimes the
female more than the male. In both, the tendency goes to a certain
extent with systematic affinity, for whole groups of animals and plants
are rendered impotent by the same unnatural conditions; and whole
groups of species tend to produce sterile hybrids. On the other hand,
one species in a group will sometimes resist great changes of
conditions with unimpaired fertility; and certain species in a group
will produce unusually fertile hybrids. No one can tell, till he tries,
whether any particular animal will breed under confinement or any plant
seed freely under culture; nor can he tell, till he tries, whether any
two species of a genus will produce more or less sterile hybrids.
Lastly, when organic beings are placed during several generations under
conditions not natural to them, they are extremely liable to vary,
which is due, as I believe, to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids in successive generations
are eminently liable to vary, as every experimentalist has observed.
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
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