inct 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 [in]
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 page 100, you will
find a somewhat analogous discussion, perhaps more intelligible than
this letter.

LETTER 289. W.T. THISELTON-DYER TO THE EDITOR OF "NATURE."

(289/1. The following letter ("Nature," Volume XLIII., page 535)
criticises the interpretation given by the Duke to Mr. Darwin's letter.)

Royal Gardens, Kew, March 27th [1891].

In "Nature" of March 5th (page 415), the Duke of Argyll has printed
a very interesting letter of Mr. Darwin's, from which he drew the
inference that the writer "assumed mankind to have arisen...in a single
pair." I do not think myself that the letter bears this interpretation.
But the point in its most general aspect is a very important one, and
is often found to present some difficulty to students of Mr. Darwin's
writings.

Quite recently I have found by accident, amongst the papers of the late
Mr. Bentham at Kew, a letter of friendly c