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Updated: May 1, 2025
In some cases I do not doubt that the intercrossing of species aboriginally distinct has played an important part in the origin of our domestic productions. p. 43. What new words are these for a loyal disciple of the true Baconian philosophy? "I can conceive" "It is not incredible" "I do not doubt" "It is conceivable."
But in the majority of cases, namely, with all organisms which habitually unite for each birth, or which often intercross, I believe that during the slow process of modification the individuals of the species will have been kept nearly uniform by intercrossing; so that many individuals will have gone on simultaneously changing, and the whole amount of modification will not have been due, at each stage, to descent from a single parent.
Or, again, the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to hunt different prey; and from the continued preservation of the individuals best fitted for the two sites, two varieties might slowly be formed. These varieties would cross and blend where they met; but to this subject of intercrossing we shall soon have to return.
We are warned that the following species are "difficult of extrication," either confluent or mixed by intercrossing. H. doronicoides. I place this the third in merit among perennial sunflowers, H. rigidus and H. multiflorus being first and second. It is 6 feet or 7 feet high, upright in growth, with many stalks.
Again, after speaking of "the general law of good being, derived from the intercrossing of distinct individuals of the same species," and the evidence that the pollen of a distinct variety or race is prepotent over a flower's own pollen, adds the very significant remark, "When distinct species are crossed, the case is directly the reverse, for a plant's own pollen is almost always prepotent over foreign pollen."
On the Intercrossing of Individuals. I must here introduce a short digression. Nevertheless I am strongly inclined to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur for the reproduction of their kind. This view was first suggested by Andrew Knight.
In domestication, this intercrossing may be prevented; and in this prevention lies the art of producing varieties. We see everywhere around us the remarkable results which Nature may be said to have brought about under artificial selection and separation. Could she accomplish similar results when left to herself? Variations might begin, we know they do begin, in a wild state.
Nothing can be effected, unless favourable variations occur, and variation itself is apparently always a very slow process. The process will often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply sufficient wholly to stop the action of natural selection. I do not believe so.
But it by no means follows, that, because in an island nearly all the species of one class are peculiar, those of another class, or of another section of the same class, are peculiar; and this difference seems to depend partly on the species which do not become modified having immigrated with facility and in a body, so that their mutual relations have not been much disturbed; and partly on the frequent arrival of unmodified immigrants from the mother-country, and the consequent intercrossing with them.
Finally, looking to all the ascertained facts on the intercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal.
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