been said on the extreme sensitiveness and delicate
affinities of the reproductive system, why should we
feel any surprise at the sexual elements of those
forms, which we call species, having been differentiated
in such a manner that they are incapable or only feebly
capable of acting on one another? We know that
species have generally lived under the same conditions,
and have retained their own proper characters, for
a much longer period than varieties. Long-continued
domestication eliminates, as I have shown in my ‘Variation
under Domestication,’ the mutual sterility which
distinct species lately taken from a state of nature
almost always exhibit when intercrossed; and we can
thus understand the fact that the most different domestic
races of animals are not mutually sterile. But
whether this holds good with cultivated varieties of
plants is not known, though some facts indicate that
it does. The elimination of sterility through
long-continued domestication may probably be attributed
to the varying conditions to which our domestic animals
have been subjected; and no doubt it is owing to this
same cause that they withstand great and sudden changes
in their conditions of life with far less loss of
fertility than do natural species. From these
several considerations it appears probable that the
difference in the affinities of the sexual elements
of distinct species, on which their mutual incapacity
for breeding together depends, is caused by their having
been habituated for a very long period each to its
own conditions, and to the sexual elements having
thus acquired firmly fixed affinities. However
this may be, with the two great classes of cases before
us, namely, those relating to the self-fertilisation
and cross-fertilisation of the individuals of the
same species, and those relating to the illegitimate
and legitimate unions of heterostyled plants, it is
quite unjustifiable to assume that the sterility of
species when first crossed and of their hybrid offspring,
indicates that they differ in some fundamental manner
from the varieties or individuals of the same species.
Abutilon darwinii, self-sterile in Brazil. moderately
self-fertile in England. fertilised by birds.
Adonis aestivalis. measurements. relative heights
of crossed and self-fertilised plants. self-fertile.
Allium cepa (blood-red var.)
Anagallis collina (var. grandiflora). measurements.
Anderson, J., on the Calceolaria.
removing the corollas.
Antirrhinum majus (red var.) perforated corolla.
—(white var.). —(peloric var.).
result of experiments.