ey, after many generations had
passed, differ? When we reflect on the cases given in the twenty-third
chapter, and on such facts as the difference in former times between the
breeds of cattle, sheep, &c., in almost every district of Europe, we are
strongly inclined to admit that the two lots would be differently modified
through the influence of climate and food. But the evidence on the definite
action of changed conditions is in most cases insufficient; and, with
respect to pigeons, I have had the opportunity of examining a large
collection of domesticated birds, sent to me by Sir W. Elliot from India,
and they varied in a remarkably similar manner with our European birds.

If two distinct breeds were to be confined together in equal numbers, there
is reason to suspect that they would to a certain extent prefer pairing
with their own kind; but they would likewise intercross. From the greater
vigour and fertility of the crossed offspring, the whole body would by this
means become interblended sooner than would otherwise have occurred. From
certain breeds being prepotent over others, it does not follow that the
interblended progeny would be strictly intermediate in character. I have,
also, proved that the act of crossing in itself gives a strong tendency to
reversion, so that the crossed offspring would tend to revert to the state
of the aboriginal rock-pigeon. In the course of time they would probably be
not much more heterogeneous in character than in our first case, when birds
of the same breed were confined together. {423}

I have just said that the crossed offspring would gain in vigour and
fertility. From the facts given in the seventeenth chapter there can be no
doubt of this; and there can be little doubt, though the evidence on this
head is not so easily acquired, that long-continued close interbreeding
leads to evil results. With hermaphrodites of all kinds, if the sexual
elements of the same individual habitually acted on each other, the closest
possible interbreeding would be perpetual. Therefore we should bear in mind
that with all hermaphrodite animals, as far as I can learn, their structure
permits and frequently necessitates a cross with a distinct individual.
With hermaphrodite plants we incessantly meet with elaborate and perfect
contrivances for this same end. It is no exaggeration to assert that, if
the use of the talons and tusks of a carnivorous animal, or the use of the
viscid threads of a spider's