linaceous birds, that the early death of the embryo is a very
frequent cause of sterility in first crosses. I was at first very
unwilling to believe in this view; as hybrids, when once born, are
generally healthy and long-lived, as we see in the case of the common
mule. Hybrids, however, are differently circumstanced before and after
birth: when born and living in a country where their two parents can
live, they are generally placed under suitable conditions of life. But
a hybrid partakes of only half of the nature and constitution of its
mother, and therefore before birth, as long as it is nourished within
its mother's womb or within the egg or seed produced by the mother, it
may be exposed to conditions in some degree unsuitable, and consequently
be liable to perish at an early period; more especially as all very
young beings seem eminently sensitive to injurious or unnatural
conditions of life.

In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is very different. I have more than once
alluded to a large body of facts, which I have collected, showing that
when animals and plants are removed from their natural conditions,
they are extremely liable to have their reproductive systems seriously
affected. This, in fact, is the great bar to the domestication of
animals. Between the sterility thus superinduced and that of hybrids,
there 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
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
cond