With plants, hybridized embryos probably
often perish in a like manner; at least it is known that 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.
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