hybrids raised from very distinct species are sometimes weak and
dwarfed, and perish at an early age; of which fact Max Wichura has
recently given some striking cases with hybrid willows. It may be here
worth noticing that in some cases of parthenogenesis, the embryos within
the eggs of silk moths which had not been fertilised, pass through their
early stages of development and then perish like the embryos produced by
a cross between distinct species. Until becoming acquainted with these
facts, I was unwilling to believe in the frequent early death of
hybrid embryos; for 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 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; it may therefore,
before birth, as long as it is nourished within its mother's womb, or
within the egg or seed produced by the mother, 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 are eminently
sensitive to injurious or unnatural conditions of life. But after all,
the cause more probably lies in some imperfection in the original act
of impregnation, causing the embryo to be imperfectly developed, rather
than in the conditions to which it is subsequently exposed.

In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is somewhat different. I have more than
once alluded to a large body of facts 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