VII 
ON THE INFERTILITY OF CROSSES 
179 
such infertility there is no proof. This is admitted; but it is 
urged that facts have been adduced which render such 
infertility probable, at least in some cases, and this is all 
that is required. It is by no means necessary that all varieties 
should exhibit incipient infertility, but only some varieties; 
for Ave know that, of the innumerable varieties that occur 
but few become developed into distinct species, and it may be 
that the absence of infertility, to obviate the effects of inter¬ 
crossing, is one of the usual causes of their failure. All I 
have attempted to shoAv is, that when incipient infertility does 
occur in correlation with other varietal differences, that in¬ 
fertility can be, and in fact must be, increased by natural 
selection ; and this, it appears to me, is a decided step in 
advance in the solution of the problem . 1 
1 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 Avith 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 propor¬ 
tion 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, iu 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, and this will further tend to the 
diminution of the production of hybrids. 
