35i 
in the few following cases, which I will briefly abstract. The evi 
dence is at least as good as that from which we believe in the 
sterility of a multitude of species. The evidence is also derived 
from hostile witnesses, who in all other cases consider fertility and 
sterility as safe criterions of specific distinction.”—(Origin of Species. 
P- 257-) 
“ The yellow and white varieties (of Verbascum), when crossed, 
produce less seed than the similarly coloured varieties of the same 
species. . . . The blue and red varieties of the pimpernel 
are absolutely sterile together, while each is perfectly fertile 
within itself.”—(Journal of Linnean Society. Zoology, vol. xix., 
P- 359)• 
“ It is most instructive to compare . . . the evidence advanced 
by our best botanists on the question whether certain doubtful forms 
should be ranked as species or varieties with the evidence from 
fertility adduced by different hybridisers, or by the same observer 
from experiments made during different years. It can thus be shown 
that neither sterility nor fertility affords any certain distinction be 
tween species and varieties. The evidence from this source graduates 
away, and is doubtful in the same degree as is the evidence derived 
from other constitutional and structural differences.”—(Origin of 
Species, p. 237.) 
On the other hand, offspring of different species are some 
times fertile inter se. 
“ Some species have a remarkable power of crossing with other 
species.” . . . “The degree of fertility both of first crosses and 
of hybrids graduates from zero to perfect fertility.”—(Origm of 
Species, pp. 244, 241.) 
“ Kolreuter makes the rule universal; but then he cuts the knot, 
for, in ten cases in which he found two forms, considered by 
most authors as distinct species, quite fertile together, he unhesi 
tatingly ranks them as varieties.” . . . “If two forms, hitherto 
reputed to be varieties, be found in any degree sterile together, 
they are at once ranked by most naturalists as species. For instance, 
the red and blue pimpernel, which are considered by most botanists 
as varieties, are said by Gartner to be quite sterile when crossed, 
and he consequently ranks them as undoubted species. If we thus 
argue in a circle, the fertility of all varieties produced under nature 
will assuredly have to be granted.”—(Origin of Species, pp. 236, 
236.)