with each other in almost their
whole organisation, yet are not rarely, some of them hermaphrodites,
and some of them unisexual. But if, in fact, all hermaphrodites do
occasionally intercross with other individuals, the difference between
hermaphrodites and unisexual species, as far as function is concerned,
becomes very small.

From these several considerations and from the many special facts which
I have collected, but which I am not here able to give, I am strongly
inclined to suspect that, both in the vegetable and animal kingdoms, an
occasional intercross with a distinct individual is a law of nature. I
am well aware that there are, on this view, many cases of difficulty,
some of which I am trying to investigate. Finally then, we may conclude
that in many organic beings, a cross between two individuals is an
obvious necessity for each birth; in many others it occurs perhaps only
at long intervals; but in none, as I suspect, can self-fertilisation go
on for perpetuity.

CIRCUMSTANCES FAVOURABLE TO NATURAL SELECTION.

This is an extremely intricate subject. A large amount of inheritable
and diversified variability is favourable, but I believe mere individual
differences suffice for the work. A large number of individuals, by
giving a better chance for the appearance within any given period
of profitable variations, will compensate for a lesser amount of
variability in each individual, and is, I believe, an extremely
important element of success. Though nature grants vast periods of time
for the work of natural selection, she does not grant an indefinite
period; for as all organic beings are striving, it may be said, to seize
on each place in the economy of nature, if any one species does
not become modified and improved in a corresponding degree with its
competitors, it will soon be exterminated.

In man's methodical selection, a breeder selects for some definite
object, and free intercrossing will wholly stop his work. But when many
men, without intending to alter the breed, have a nearly common standard
of perfection, and all try to get and breed from the best animals,
much improvement and modification surely but slowly follow from this
unconscious process of selection, notwithstanding a large amount of
crossing with inferior animals. Thus it will be in nature; for within a
confined area, with some place in its polity not so perfectly occupied
as might be, natural selection will always tend to preserve all the