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Updated: June 1, 2025
In some cases the intercrossing of aboriginally distinct species appears to have played an important part in the origin of our breeds.
Whether such variations or individual differences as may arise will be accumulated through natural selection in a greater or less degree, thus causing a greater or less amount of permanent modification, will depend on many complex contingencies on the variations being of a beneficial nature, on the freedom of intercrossing, on the slowly changing physical conditions of the country, on the immigration of new colonists, and on the nature of the other inhabitants with which the varying species come into competition.
Dozens of subspecies may be cultivated in the same garden without any real danger of their intercrossing. They remain as pure as under perfect isolation. It is very interesting to observe the aspect of such types, when growing near each other. Hundreds of rosettes exhibit one type, and are undoubtedly similar.
If reproductive elements were confined to one special part or organ, we should be unable to explain the reproduction of lost limbs in salamanders, and the persistent effect of intercrossing on subsequent issue by the same mother, and the propagation of plants from shoots, or of the begonia from minute fragments of leaves, or the development of small pieces of water-worms into complete animals.
But in the great majority of cases, namely, with all organisms which habitually unite for each birth, or which occasionally intercross, the individuals of the same species inhabiting the same area will be kept nearly uniform by intercrossing; so that many individuals will go on simultaneously changing, and the whole amount of modification at each stage will not be due to descent from a single parent.
Owl-laced Night Apes A third interesting genus of monkeys found near Ega, are the Nyctipitheci, or night apes, called Ei-a by the Indians. Of these I found two species, closely related to each other but nevertheless quite distinct, as both inhabit the same forests, namely, those of the higher and drier lands, without mingling with each other or intercrossing.
Moreover, there is the chance of small changes being lost by intercrossing: our own cattle-breeders have most carefully to select the parents, or else the favourable variety soon disappears.
When several breeds have once been formed in any country, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of crossing has been much exaggerated, both in regard to animals and to those plants which are propagated by seed.
Intercrossing plays a very important part in nature by keeping the individuals of the same species, or of the same variety, true and uniform in character. It will obviously thus act far more efficiently with those animals which unite for each birth; but, as already stated, we have reason to believe that occasional intercrosses take place with all animals and plants.
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. Laws governing the Sterility of first Crosses and of Hybrids.
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