Yet further, we have seen that if several competing species in the same area were being simultaneously modified, those between whose varieties infertility arose would have an advantage over those whose varieties remained fertile _inter se_, and would ultimately supplant them.
The preceding argument, it will be seen, depends entirely upon the assumption that some amount of infertility characterises the distinct varieties which are in process of differentiation into species; and it may be objected that of 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 we 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 intercrossing, is one of the usual causes of their failure.

All I have attempted to show is, that _when_ incipient infertility does occur in correlation with other varietal differences, that infertility 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.[62] _Physiological Selection._ Another form of infertility has been suggested by Professor G.J.

Romanes as having aided in bringing about the characteristic infertility or sterility of hybrids.