AT MAJORITY of the individuals will there consist of the pure forms
only, while in the rest of the area these will be in a minority,--which
is the same as saying, that the new sterile or physiological variety of
the two forms will be better suited to the conditions of existence than
the remaining portion which has not varied physiologically.

9. But when the struggle for existence becomes severe, that variety
which is best adapted to the conditions of existence always supplants
that which is imperfectly adapted; therefore by Natural Selection the
sterile varieties of the two forms will become established as the only
ones.

10. Now let a fresh series of variations in the amount of sterility and
in the disinclination to crossed unions occur,--also in certain parts
of the area: exactly the same result must recur, and the progeny of this
new physiological variety again in time occupy the whole area.

11. There is yet another consideration that supports this view. It seems
probable that the variations in amount of sterility would to some extent
concur with and perhaps depend upon the structural variations; so that
just in proportion as the two forms diverged and became better adapted
to the conditions of existence, their sterility would increase. If this
were the case, then Natural Selection would act with double strength,
and those varieties which were better adapted to survive both
structurally and physiologically, would certainly do so. (211/3. The
preceding eleven paragraphs are substantially but not verbally identical
with the statement of the argument in Mr. Wallace's "Darwinism," 1889.
Pages 179, 180, note 1.)

12. Let us now consider the more difficult case of two allied species
A, B, in the same area, half the individuals of each (As, Bs) being
absolutely sterile, the other half (Af, Bf) being partially fertile:
will As, Bs ultimately exterminate Af, Bf?

13. To avoid complication, it must be granted, that between As and Bs
no cross-unions take place, while between Af and Bf cross-unions are as
frequent as direct unions, though much less fertile. We must also leave
out of consideration crosses between As and Af, Bs and Bf, with their
various approaches to sterility, as I believe they will not affect the
final result, although they will greatly complicate the problem.

14. In the first generation there will result: 1st, The pure progeny of
As and Bs; 2nd, The pure progeny of Af and of Bf; and 3rd, The hybrid
pr