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Darwin on the Unity of the Human Race.

Having had occasion last year to allude as a fact to the circumstance that Charles Darwin assumed mankind to have arisen at one place, and therefore in a single pair, I was surprised to find that this fact was doubted, or at least very doubtfully accepted, by some of my scientific friends; and I was asked for a reference to his works in confirmation of it. My principal reliance, however, was in the recollection of a private letter to myself from the illustrious naturalist, which I had unfortunately mislaid. Having now recovered this letter, I send a copy of it to Nature for publication, simply explaining that this letter was in reply to a letter from me in which I put the direct question, why it was that he did assume the unity of mankind as descended from a single pair? It will be observed that in his reply he does not repudiate this interpretation of his theory, but simply proceeds to explain and to defend the doctrine.

ARGYLL.

"Down, Beckenham, September 23, 1878.

"DEAR DUKE OF ARGYLL,—The problem which you state so clearly is a very interesting one, on which I have often speculated. As far as I can judge, the improbability is extreme that the same well-characterized species should be produced in two distinct countries, or at two distinct times. It is certain that the same variation may arise in two distinct places, as with albinism or with the nectarine on peach-trees. But the evidence seems to me overwhelming that a well-marked species is the product, not of a single or of a few variations, but of a long series of modifications, each modification resulting chiefly from adaptation to infinitely complex conditions (including the inhabitants of the same country) with more or less inheritance of all the preceding modifications. Moreover, as variability depends more on the nature of the organism than on that of the environment, the variations will tend to differ at each successive stage of descent. Now it seems to me improbable in the highest degree that a species should ever have been exposed in two places to infinitely complex relations of exactly the same nature during a long series of modifications. An illustration will perhaps make what I have said clearer, though it applies only to the less important factors of inheritance and variability, and not to adaptation—viz. the improbability of two men being born in two countries identical in body and mind. If, however, it be assumed that a species at each successive stage of its modification was surrounded in two distinct countries or times by exactly the same assemblage of plants and animals, and by the same physical conditions, then I can see no theoretical difficulty to such a species giving birth to the new form in the two countries. If you will look to the sixth edition of my 'Origin,' at p. 100, you will find a somewhat analogous discussion perhaps more.

"Yours faithfully,

"Charles Darwin."