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Updated: May 17, 2025
But we often take, I think, an erroneous view of the probability of closely-allied species invading each other's territory, when put into free intercommunication.
When the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards it, the contrivance seems adapted solely to ensure self-fertilisation; and no doubt it is useful for this end: but, the agency of insects is often required to cause the stamens to spring forward, as Kölreuter has shown to be the case with the barberry; and in this very genus, which seems to have a special contrivance for self-fertilisation, it is well known that if closely-allied forms or varieties are planted near each other, it is hardly possible to raise pure seedlings, so largely do they naturally cross.
These remarks likewise apply to the nuptial or summer plumage, whether confined to the males, or common to both sexes. Although the females of the above closely-allied or representative species, together with their young, differ hardly at all from one another, so that the males alone can be distinguished, yet the females of most species within the same genus obviously differ from each other.
If then, as appears probable, species first become rare and then extinct if the too rapid increase of every species, even the most favoured, is steadily checked, as we must admit, though how and when it is hard to say and if we see, without the smallest surprise, though unable to assign the precise reason, one species abundant and another closely-allied species rare in the same district why should we feel such great astonishment at the rarity being carried a step farther to extinction?
Close observers find among the aboriginal tribes resident far up on the north-west American coast peculiar habits and customs, having closely-allied types among the Chinese. The features of the Aleuts, the natives of the Aleutian Islands, are said to approximate closely to those of the Mongolians.
Moritz Wagner has shown that, in the case of insects, a moderately- broad river may divide two closely-allied species of beetles, or a very narrow snow-range, two closely-allied species of moths.
The hare on her form is a familiar instance of concealment through colour; yet this principle partly fails in a closely-allied species, the rabbit, for when running to its burrow, it is made conspicuous to the sportsman, and no doubt to all beasts of prey, by its upturned white tail.
With many closely-allied species, following nearly the same habits of life, the males have come to differ from each other chiefly through the action of sexual selection; whilst the females have come to differ chiefly from partaking more or less of the characters thus acquired by the males.
However this may be, I have given in my 'Descent of Man' instances of closely-allied birds inhabiting distinct countries, of which the young and the females cannot be distinguished, whilst the adult males differ considerably, and this may be attributed with much probability to the action of sexual selection. Abbot, C., on the battles of seals.
In the Galapagos Archipelago, many even of the birds, though so well adapted for flying from island to island, are distinct on each; thus there are three closely-allied species of mocking-thrush, each confined to its own island.
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