ere 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. It is founded on the fact, already noticed, that
certain individuals of some species possess what may be termed selective
sterility--that is, while fertile with some individuals of the species
they are sterile with others, and this altogether independently of any
differences of form, colour, or structure. The phenomenon, in the only
form in which it has been observed, is that of "infertility or absolute
sterility between two individuals, each of which is perfectly fertile
with all other individuals;" but Mr. Romanes thinks that "it would not
be nearly so remarkable, or physiologically improbable, that such
incompatibility should run through a whole race or strain."[63]
Admitting that this may be so, though we have at present no evidence
whatever in support of it, it remains to be considered whether such
physiological varieties could maintain themselves, or whether, as in the
cases of sporadic infertility already discussed, they would necessarily
die out unless correlated wi