ogeny of Af, Bf.

15. Supposing that, in ordinary years, the increased constitutional
vigour of the hybrids exactly counterbalances their imperfect
adaptations to conditions, there will be in the second generation,
besides these three classes, hybrids of the second degree between the
first hybrids and Af and Bf respectively. In succeeding generations
there will be hybrids of all degrees, varying between the first hybrids
and the almost pure types of Af and Bf.

16. Now, if at first the number of individuals of As, Bs, Af and Bf
were equal, and year after year the total number continues stationary,
I think it can be proved that, while half will be the pure progeny of As
and Bs, the other half will become more and more hybridised, until the
whole will be hybrids of various degrees.

17. Now, this hybrid and somewhat intermediate race cannot be so well
adapted to the conditions of life as the two pure species, which have
been formed by the minute adaptation to conditions through Natural
Selection; therefore, in a severe struggle for existence, the hybrids
must succumb, especially as, by hypothesis, their fertility would not be
so great as that of the two pure species.

18. If we were to take into consideration the unions of As with Af and
Bs with Bf, the results would become very complicated, but it must still
lead to there being a number of pure forms entirely derived from As and
Bs, and of hybrid forms mainly derived from Af and Bf; and the result of
the struggle of these two sets of individuals cannot be doubtful.

19. If these arguments are sound, it follows that sterility may
be accumulated and increased, and finally made complete by Natural
Selection, whether the sterile varieties originate together in a
definite portion of the area occupied by the two species, or occur
scattered over the whole area. (211/4. The first part of this discussion
should be considered alone, as it is both more simple and more
important. I now believe that the utility, and therefore the cause of
sterility between species, is during the process of differentiation.
When species are fully formed, the occasional occurrence of hybrids
is of comparatively small importance, and can never be a danger to the
existence of the species. A.R.W. (1899).)

P.S.--In answer to the objection as to the unequal sterility of
reciprocal crosses ("Variation, etc." Volume II., page 186) I
reply that, as far as it went, the sterility of one cross would be
ad