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-)