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 with useful characters. Mr. Romanes thinks
that they would persist, and urges that "whenever this one kind of
variation occurs _it cannot escape the preserving agency_ of
physiological selection. Hence, even if it be granted that the variation
which affects the reproductive system in this particular way is a
variation of comparatively rare occurrence, still, as _it must always be
preserved_ whenever it does occur, its influence in the manufacture of
specific types _must be cumulative_.
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