i87o— 1882] DUKE OF ARGYLL 377 



much about my election to the Institute, but the sympathy of Letter 287 

 some few of my friends has gratified me deeply. 



I am extremely glad to hear that you are going to publish 

 a work on the more ancient fossil plants ; and I thank you 

 beforehand for the volume which you kindly say that you 

 will send me. I earnestly hope that you will give, at least 

 incidentally, the results at which you have arrived with 

 respect to the more recent Tertiary plants ; for the close 

 gradation of such forms seems to me a fact of paramount 

 importance for the principle of evolution. Your cases are 

 like those on the gradation in the genus Equus, recently 

 discovered by Marsh in North America. 



To the Duke of Argyll. Letter 288 



The following letter was published in Nature, March 5th, 1891, Vol. 

 XLIII., p. 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, Sept. 23rd, 1878. 

 The problem which you state so clearly is a very inter- 

 esting 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 modifi- 

 cation resulting chiefly from adaptation to infinitely complex 

 conditions (including the inhabitants of the same country), 

 with more or less inheritance of all the preceding modifica- 

 tions. 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 



