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 seems to be
partly due to their reproductive systems having been specially affected,
though in a lesser degree than when sterility ensues. So it is with
hybrids, for their offspring 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 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 distinct structures and
constitutions, including of course the reproductive systems, 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 relations 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, does not diminish; it is even apt to increase,
this being generally the result, as before explained, of too close
interbreeding. The above view of the sterility of hybrids being caused
by two constitutions being compounded into one has been strongly
maintained by Max Wichura.

It must, however, be owned that we cannot understand, on the above or
any other view, several facts with r