stic races differ so
greatly in size (as the great stag-greyhound and lap-dog, or cart-horse
and Burmese ponies) that union is nearly impossible; and what is less
generally known is, that in plants Koelreuter has shown by hundreds of
experiments that the pollen of one species will fecundate the germen of
another species, whereas the pollen of this latter will never act on the
germen of the former; so that the simple fact of mutual impregnation
certainly has no relation whatever to the distinctness in creation of
the two forms. When two species are attempted to be crossed which are so
distantly allied that offspring are never produced, it has been observed
in some cases that the pollen commences its proper action by exserting
its tube, and the germen commences swelling, though soon afterwards it
decays. In the next stage in the series, hybrid offspring are produced
though only rarely and few in number, and these are absolutely sterile:
then we have hybrid offspring more numerous, and occasionally, though
very rarely, breeding with either parent, as is the case with the common
mule. Again, other hybrids, though infertile _inter se_, will breed
_quite_ freely with either parent, or with a third species, and will
yield offspring generally infertile, but sometimes fertile; and these
latter again will breed with either parent, or with a third or fourth
species: thus Koelreuter blended together many forms. Lastly it is now
admitted by those botanists who have longest contended against the
admission, that in certain families the hybrid offspring of many of the
species are sometimes perfectly fertile in the first generation when
bred together: indeed in some few cases Mr Herbert{245} found that the
hybrids were decidedly more fertile than either of their pure parents.
There is no way to escape from the admission that the hybrids from some
species of plants are fertile, except by declaring that no form shall be
considered as a species, if it produces with another species fertile
offspring: but this is begging the question{246}. It has often been
stated that different species of animals have a sexual repugnance
towards each other; I can find no evidence of this; it appears as if
they merely did not excite each others passions. I do not believe that
in this respect there is any essential distinction between animals and
plants; and in the latter there cannot be a feeling of repugnance.

{243} <Note in the original.> If domestic