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Our old friend Midas ursulus, of Para and the Lower Amazons, is not found on the Upper river, but in its stead a closely-allied species presents itself, which appears to be the Midas rufoniger of Gervais, whose mouth is bordered with longish white hairs.
And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms, varieties of the same species, and species of the same genus or of related genera, which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other.
And it will, I believe, be universally found to be true, that wherever in two regions, let them be ever so distant, many closely-allied or representative species occur, there will likewise be found some identical species, showing, in accordance with the foregoing view, that at some former period there has been intercommunication or migration between the two regions.
And here we have the example of two very closely-allied species which cannot bear each other not because they live upon nearly the same food, as like cases are too often explained, but most probably because the passionate, eminently-individualist hare cannot make friends with that placid, quiet, and submissive creature, the rabbit.
We have every reason to believe that many of these doubtful and closely-allied forms have permanently retained their characters in their own country for a long time; for as long, as far as we know, as have good and true species.
And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms, varieties of the same species, and species of the same genus or of related genera, which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other.
It would be almost superfluous to adduce evidence in support of the statement, that ordinary specific characters are more variable than generic; but with respect to important characters, I have repeatedly noticed in works on natural history, that when an author remarks with surprise that some important organ or part, which is generally very constant throughout a large group of species, DIFFERS considerably in closely-allied species, it is often VARIABLE in the individuals of the same species.
It is known that for a long time Darwin was worried by the difficulty which he saw in the absence of a long chain of intermediate forms between closely-allied species, and that he found the solution of this difficulty in the supposed extermination of the intermediate forms.
The fragment on the “Mathematical” or “Geometric Mind” was, with the exception of a brief passage given by Des Molets in 1728, originally published, although with numerous suppressions, in Condorcet’s edition of the ‘Pensées.’ It appeared for the first time in its complete form, and under its proper title, in Faugère’s edition, along with its natural pendant, the closely-allied fragment, entitled “L’Art de Persuader.” We give a few passages from the first fragment:—
A. R. Wallace's formula concerning the origin of species, that they "have come into existence coincident both in time and place with preA"xisting closely-allied species," may or may not be true so far as individual localization is concerned.
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