are many points
of 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 exotic 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 {266} 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
steril