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. 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 criticism from Mr. Darwin upon

the presidential address which Mr. Bentham delivered to the Linnean

Society on May 24th, 1869. This letter, I think, has been overlooked and

not published previously. In it Mr. Darwin expresses himself with regard

to the multiple origin of races and some other points in very explicit

language. Prof. Meldola, to whom I mentioned in conversation the

existence of the letter, urged me strongly to print it. This, therefore,

I now do, with the addition of a few explanatory notes.

LETTER 290. TO G. BENTHAM. Down, November 25th, 1869.

(290/1. The notes to this letter are by Sir W. Thiselton-Dyer, and

appeared in "Nature," loc. cit.)

I was greatly interested by your address, which I have now read thrice,

and which I believe will have much influence on all who read it. But you

are mistaken in thinking that I ever said you were wrong on any point.

All that I meant was that on certain points, and these very doubtful

points, I was inclined to differ from you. And now, on further

considering the point on which some two or three months ago I felt most

inclined to differ--viz., on isolation--I find I differ very little.

What I have to say is really not worth saying, but as I should be very

sorry not to do whatever you asked, I will scribble down the slightly

dissentient thoughts which have occurred to me. It would be an endless

job to specify the points in which you have interested me; but I may

just mention the relation of the extreme western flora of Europe (some

such very vague thoughts have crossed my mind, relating to the Glacial

period) with South Africa, and your remarks on the contrast of passive

and active distribution.

Page lxx.--I think the contingency of a rising island, not as yet fully

stocked with plants, ought always to be kept in mind when speaking of

colonisation.

Page lxxiv.--I have met with nothing which makes me in the least doubt

that large genera present a greater number of varieties relatively to

their size than do small genera. (290/2. Bentham thought "degree of

variability... like other constitutional characters, in the first place

an individual one, which...may become more or less hereditary, and

therefore specific; and thence, but in a very faint degree, generic."

He seems to mean to argue against the conclusion which Sir Joseph

Hooker had quoted from Mr. Darwin that "species of large genera are

more variable than those of small." [On large genera varying, see Letter

53.]) Hooker was convinced by my data, never as yet published in full,

only abstracted in the "Origin."

Page lxxviii.--I dispute whether a new race or species is necessarily,

or even generally, descended from a single or pair of parents. The

whole body of individuals, I believe, become altered together--like

our race-horses, and like all domestic breeds which are changed through

"unconscious selection" by man. (290/3. Bentham had said: "We must also

admit that every race has probably been the offspring of one parent or

pair of parents, and consequently originated in one spot." The Duke of

Argyll inverts the proposition.)

When such great lengths of time are considered as are necessary to

change a specific form, I greatly doubt whether more or less rapid

powers of multiplication have more than the most insignificant weight.

These powers, I think, are related to greater or less destruction in

early life.

Page lxxix.--I still think you rather underrate the importance of

isolation. I have come to think it very important from various grounds;

the anomalous and quasi-extinct forms on islands, etc., etc., etc.

With respect to areas with numerous "individually durable" forms, can it

be said that they generally present a "broken" surface with "impassable

barriers"? This, no doubt, is true in certain cases, as Teneriffe. But

does this hold with South-West Australia or the Cape? I much doubt.

I have been accustomed to look at the cause of so many forms as being

partly an arid or dry climate (as De Candolle insists) which indirectly

leads to diversified [?] conditions; and, secondly, to isolation from

the rest of the world during a very long period, so that other more

dominant forms have not entered, and there has been ample time for much

specification and adaptation of character.

Page lxxx.--I suppose you think that the Restiaceae, Proteaceae (290/4.

It is doubtful whether Bentham did think so. In his 1870 address he

says: "I cannot resist the opinion that all presumptive evidence is

against European Proteaceae, and that all direct evidence in their

favour has broken down upon cross-examination."), etc., etc., once

extended over the world, leaving fragments in the south.

You in several places speak of distribution of plants as if exclusively

governed by soil and climate. I know that you do not mean this, but I

regret whenever a chance is omitted of pointing out that the struggle

with other plants (and hostile animals) is far more important.

I told you that I had nothing worth saying, but I have given you my

THOUGHTS.

How detestable are the Roman numerals! why should not the President's

addresses, which are often, and I am sure in this case, worth more than

all the rest of the number, be paged with Christian figures?

LETTER 291. TO R. MELDOLA.

(291/1. "This letter was in reply to a suggestion that in his preface

Mr. Darwin should point out by references to "The Origin of Species"

and his other writings how far he had already traced out the path which

Weismann went over. The suggestion was made because in a great many of

the continental writings upon the theory of descent, many of the points

which had been clearly foreshadowed, and in some cases even explicitly

stated by Darwin, had been rediscovered and published as though

original. In the notes to my edition of Weismann I have endeavoured to

do Darwin full justice.--R.M." See Letter 310.)

4, Bryanston Street, November 26th, 1878.

I am very sorry to say that I cannot agree to your suggestion. An author

is never a fit judge of his own work, and I should dislike extremely

pointing out when and how Weismann's conclusions and work agreed with my

own. I feel sure that I ought not to do this, and it would be to me

an intolerable task. Nor does it seem to me the proper office of the

preface, which is to show what the book contains, and that the contents

appear to me valuable. But I can see no objection for you, if you think

fit, to write an introduction with remarks or criticisms of any kind. Of

course, I would be glad to advise you on any point as far as lay in my

power, but as a whole I could have nothing to do with it, on the grounds

above specified, that an author cannot and ought not to attempt to judge

his own works, or compare them with others. I am sorry to refuse to do

anything which you wish.

LETTER 292. TO T.H. HUXLEY. Down, January 18th, 1879.

I have just finished your present of the Life of Hume (292/1. "Hume" in

Mr. Morley's "English Men of Letters" series. Of the biographical part

of this book Mr. Huxley wrote, in a letter to Mr. Skelton, January 1879

("Life of T.H. Huxley," II., page 7): "It is the nearest approach to a

work of fiction of which I have yet been guilty."), and must thank you

for the great pleasure which it has given me. Your discussions are,

as it seems to me, clear to a quite marvellous degree, and many of

the little interspersed flashes of wit are delightful. I particularly

enjoyed the pithy judgment in about five words on Comte. (292/2.

Possibly the passage referred to is on page 52.) Notwithstanding the

clearness of every sentence, the subjects are in part so difficult that

I found them stiff reading. I fear, therefore, that it will be too stiff

for the general public; but I heartily hope that this will prove to be a

mistake, and in this case the intelligence of the public will be greatly

exalted in my eyes. The writing of this book must have been awfully hard

work, I should think.

LETTER 293. TO F. MULLER. Down, March 4th [1879].

I thank you cordially for your letter. Your facts and discussion on the

loss of the hairs on the legs of the caddis-flies seem to me the most

important and interesting thing which I have read for a very long time.

I hope that you will not disapprove, but I have sent your letter to

"Nature" (293/1. Fritz Muller, "On a Frog having Eggs on its Back--On

the Abortion of the Hairs on the Legs of certain Caddis-Flies, etc.":

Muller's letter and one from Charles Darwin were published in "Nature,"