. 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 {264} 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 {265} 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