3§4
can only breed with one another will be modified in the
same way, and so will constitute in a very short time an
incipient species.
Another case of physiological differentiation is to be
found in those cases (if there be such) in which a group of
individuals vary from the rest of the species, and are also
infertile with those from whom they differ, while they are
perfectly fertile inter se.
Mr. Thomas Belt says :—
“ The varieties that arise can seldom be separated from the parent
form and from other varieties until they vary also in the elements
of reproduction. Thousands of varieties probably revert to the
parent type, but if, at last, one is produced that breeds only with its
own form, we can easily see how a new series might be segregated.”
—(The Naturalist in Nicaragua, p. 20J.)
Mr. Romanes says :—
“ If the variation be such that the reproductive system, while
showing some degree of sterility with the parent form, continues
to be fertile within the limits of the varietal form, in this case the
variation would neither be swamped by intercrossing nor would it
die out on account of sterility. On the contrary, the variation would
be perpetuated with more certainty than could a variation of any
other kind. For, in virtue of increased sterility with the parent
form, the variation would not be exposed to extinction by inter
crossing ; while, in virtue of continued fertility within the varietal
form, the variation would perpetuate itself by heredity, just as in
the case of variations generally when not re-absorbed by inter
crossing.”—(Journal of the Linnean Society—Zoology, vol. xix.,
A 352)•
That this is not a mere idle supposition is seen from the
fact that, according to Gärtner,—
“The yellow and white varieties (of Verbascum), when crossed,
produce less seed than the similarly coloured varieties of the same
species ; and . . . that the blue and red varieties of the pim
pernel are absolutely sterile together, while each is perfectly fertile
within itself.”—{Apud Romanes. Journal of the Linnean Society—
Zoology, vol. xix., p. JS9-)