at is sufficient proof that the
slightest variation in that direction would be useful also, and would go
on accumulating.

1. Let there be a species which has varied into two forms, each adapted
to existing conditions (211/1. "Existing conditions," means of course
new conditions which have now come into existence. And the "two" being
both better adapted than the parent form, means that they are better
adapted each to a special environment in the same area--as one to damp,
another to dry places; one to woods, another to open grounds, etc.,
etc., as Darwin had already explained. A.R.W. (1899).) better than the
parent form, which they supplant.

2. If these two forms, which are supposed to co-exist in the same
district, do not intercross, Natural Selection will accumulate
favourable variations, till they become sufficiently well adapted to
their conditions of life and form two allied 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. (211/2. After "pure breeds," add
"because less specialised." A.R.W. (1899).)

4. Now let a partial sterility of some individuals of these two forms
arise when they intercross; 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 is that in this area hybrids will not increase so rapidly
as before; and as by the terms of the problem the two pure forms are
better suited to the conditions of life than the hybrids, they will tend
to supplant the latter altogether whenever the struggle for existence
becomes severe.

6. We may fairly suppose, also, that as soon as any sterility appears
under natural conditions, it will be accompanied by some disinclination
to cross-unions; and this will further diminish the production of
hybrids.

7. In the other part of the area, however, where hybridism occurs
unchecked, hybrids of various degrees will soon far outnumber the parent
or pure form.

8. The first result, then, of a partial sterility of crosses appearing
in one part of the area occupied by the two forms, will be, that the
GRE