252 APPENDIX A 



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 modi- 

 fication 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 

 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.' ' 



The Duke misunderstood the letter, for he used 

 it as evidence to support his assertion ' that 

 Charles Darwin assumed mankind to have arisen 

 at one place, and therefore in a single pair '. 

 It is obvious that no such conclusion follows 

 from Darwin's argument ; but in order to settle 

 the question once for all, Sir William Thiselton- 

 Dyer published a letter 2 in which Darwin makes 

 the following statement : 



1 Nature, xliii. 415. At the conclusion of the letter Darwin 

 refers his correspondent to p. 100 of the sixth ed. of the Origin. See 

 also More Letters, i. 377, 378. 



* Nature, xliii. 535. See also More Letters, i. 378-81. 



