for, as just remarked, in the union of two pure species the male and
female sexual elements are perfect, whereas in hybrids they are
imperfect. Even in first crosses, the greater or lesser difficulty in
effecting a union apparently depends on several distinct causes. There
must sometimes be a physical impossibility in the male element reaching
the ovule, as would be the case with a plant having a pistil too long
for the pollen-tubes to reach the ovarium. It has also been observed
that when pollen of one species is placed on the stigma of a distantly
allied species, though the pollen-tubes protrude, they do not penetrate
the stigmatic surface. Again, the
male element may reach the female element, but be incapable of causing
an embryo to be developed, as seems to have been the case with some of
Thuret s experiments on Fuci. No explanation can be given of these
facts, any more than why certain trees cannot be grafted on others.
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. 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
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