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.
INDEX.
Abutilon darwinii, self-sterile in Brazil.
moderately self-fertile in England.
fertilised by birds.
Acacia sphaerocephala.
Acanthaceae.
Aconitum napellus.
Adlumia cirrhosa.
Adonis aestivalis.
measurements.
relative heights of crossed and self-fertilised plants.
self-fertile.
Ajuga reptans.
Allium cepa (blood-red var.)
Anagallis collina (var. grandiflora).
measurements.
seeds.
Anderson, J., on the Calceolaria.
removing the corollas.
Anemone.
Anemophilous plants.
often diclinous.
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