138 EVOLUTION [Chap. Ill 



Letter 92 rests on very slight evidence, yet I think is very probably 

 true, as may be inferred from the case of dogs. Under 

 nature it seems improbable that the differences in the repro- 

 ductive constitution, on which the sterility of any two species 

 when crossed depends, can be acquired directly by Natural 

 Selection ; for it is of no advantage to the species. Such 

 differences in reproductive constitution must stand in cor- 

 relation with some other differences ; but how impossible to 

 conjecture what these are ! Reflect on the case of the 

 variations of Vcrbasatm, which differ in no other respect 

 whatever besides the fluctuating element of the colour of the 

 flower, and yet it is impossible to resist Gartner's evidence, 

 that this difference in the colour does affect the mutual 

 fertility of the varieties. 



The whole case seems to me far too mysterious to rest ' a 

 valid attack on the theory of modification of species, though, 

 as you say, it offers excellent ground for a mere advocate. 



I am surprised, considering how ignorant we are on 

 very many points, [that] more weak parts in my book have 

 not as yet been pointed out to me. No doubt many will be. 

 H. C. Watson founds his objection in MS. on there being no 

 limit to infinite diversification of species : I have answered this, 

 I think, satisfactorily, and have sent attack and answer to 

 Lyell and Hooker. If this seems to you a good objection, 

 I would send papers to you. Andrew Murray " disposes of" 

 the whole theory by an ingenious difficulty from the distri- 

 bution of blind cave insects ; 2 but it can, I think, be fairly 

 answered. 



conditions of life, so that the sexual constitution of species A is attuned, 

 as it were, to a condition different from that to which B is attuned, and this 

 leads to sterility. But domestic varieties are not strictly adapted by 

 Natural Selection to definite conditions, and thus have less specialised 

 sexual constitutions. 



1 The word "rest" seems to be used in place of "to serve as a 

 foundation for." 



2 See L.ife and Letters, Vol. II., p. 265. The reference here is to 

 Murray's address before the Botanical Society, Edinburgh. Mr. Darwin 

 seems to have read Murray's views only in a separate copy reprinted 

 from the Proc. R. Soc. Edin. There is some confusion about the date 

 of the paper ; the separate copy is dated Jan. 16th, while in the volume 

 of the Proc. R. Soc. it is Feb. 20th. In the Life and Letters, II., p. 261 

 it is erroneously stated that these arc two different papers, 



