ote 60: For all these facts, see _Animals and Plants under
Domestication_, vol. ii. pp. 335-338.]

[Footnote 61: _Animals and Plants under Domestication_, vol. ii. pp.
102, 103.]

[Footnote 62: As this argument is a rather difficult one to follow,
while its theoretical importance is very great, I add here the following
briefer exposition of it, in a series of propositions; being, with a few
verbal alterations, a copy of what I wrote on the subject about twenty
years back. Some readers may find this easier to follow than the fuller
discussion in the text:--

_Can Sterility of Hybrids have been Produced by Natural
Selection?_

1. Let there be a species which has varied into _two forms_ each
adapted to certain existing conditions better than the parent
form, which they soon supplant.

2. If these _two forms_, which are supposed to coexist in the
same district, do not intercross, natural selection will
accumulate all favourable variations till they become well
suited to their conditions of life, and form two slightly
differing species.

3. But if these _two forms_ freely intercross with each other,
and produce hybrids, which are also quite fertile _inter se_,
then the formation of the two distinct races or species will be
retarded, or perhaps entirely prevented; for the offspring of
the crossed unions will be _more vigorous_ owing to the cross,
although _less adapted_ to their conditions of life than either
of the pure breeds.

4. Now, let a partial sterility of the hybrids of some
considerable proportion of these two forms arise; and, as this
would probably be due to some special conditions of life, we may
fairly suppose it to arise in some definite portion of the area
occupied by the two forms.

5. The result will be that, in that area, the hybrids (although
continually produced by first crosses almost as freely as
before) will not themselves increase so rapidly as the two pure
forms; and as the two pure forms are, by the terms of the
problem, better suited to their several conditions of life than
the hybrids, they will inevitably increase more rapidly, and
will continually tend to supplant the hybrids altogether at
every recurrent severe struggle for existence.

6. We may fairly suppose, also, that as soon as any sterility
appears some disinclination to _cross unions_ will appear,