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.

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.

Antirrhinum majus (red var.) perforated corolla. —­(white var.). —­(peloric var.).

Apium petroselinum.
result of experiments.

Argemone ochroleuca.