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.
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