Origin of Species Variorum

through natural selection; for it could .. have been of no direct advantage to an individual animal to breed badly with another individual of a different variety, and thus to leave few offspring; consequently such individuals could not have been preserved or selected. Or take the case of two species which in their present state when crossed, produce few and sterile offspring; now, what is there which could favour the survival of those individuals which happened to be endowed in a slightly higher degree with mutual infertility, and which thus approached by one small step towards absolute sterility? Yet an advance of this kind, if the theory of natural selection be brought to bear, must have incessantly occurred with many species, for a multitude are mutually quite barren. With sterile neuter insects we have reason to believe that modifications in their structure and fertility have been slowly accumulated by natural selection, from an advantage having been thus indirectly given to the community to which they belonged over other communities of the same species; but an individual animal not belonging to a social community, if rendered slightly sterile when crossed with some other variety, would not thus itself gain any advantage or indirectly give any advantage to the other individuals of the same variety, thus leading to their preservation. From these considerations I infer, as far as animals are concerned, that the various degrees of lessened fertility which occur with species when crossed cannot have been slowly accumulated by means of natural selection. ↑

With plants, it is possible that the case may be some-what different. With .. many kinds, insects constantly carry pollen from neighbouring plants to the ... stigmas of each flower; and with some species this is effected by the wind. Now if the pollen of a variety, when deposited on the stigma of the same variety, should become by spontaneous variation in ever so slight