Lastly, an embryo may be developed, and then perish at an early
period. This latter alternative has not been sufficiently attended to;
but I believe, from observations communicated to me by Mr. Hewitt, who
has had great experience in hybridising gallinaceous 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.
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