TO THE DUKE OF ARGYLL.
(288/1.

The following letter was published in "Nature," March 5th, 1891, Volume XLIII., page 415, together with a note from the late Duke of Argyll, in which he stated that the letter had been written to him by Mr.Darwin in reply to the question, "why it was that he did assume the unity of mankind as descended from a single pair." The Duke added that in the reply Mr.Darwin "does not repudiate this interpretation of his theory, but simply proceeds to explain and to defend the doctrine." On a former occasion the Duke of Argyll had "alluded as a fact to the circumstance that Charles Darwin assumed mankind to have arisen at one place, and therefore in a single pair." The letter from Darwin was published in answer to some scientific friends, who doubted the fact and asked for the reference on which the statement was based.) Down, September 23rd, 1878.
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-characterised 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.