able 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. {102}

_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 individuals varying
in the right direction, though in different degrees, so as better to fill
up the unoccupied place. But if the area be large, its several districts
will almost certainly present different conditions of life; and then if
natural selection be modifying and improving a species in the several
districts, there will be intercrossing with the other individuals of the
same species on the confines of each. And in {103} this case the effects of
intercrossing can hardly be counterbalanced by nat