298 EVOLUTION [CHAP. IV 



Letter 215 But I do not believe that you will find it so difficult. 

 When you come to Down I shall be very curious to hear 

 what your ideas are on the subject. 



Could you make anything out of a history of the great 

 steps in the progress of Botany, as representing the whole 

 of Natural History? Heaven protect you ! I suppose there 

 are men to whom such a job would not be so awful as 



it appears to me If you had time, you ought to 



read an article by W. Bagehot in the April number of 

 the Fortnightly? applying Natural Selection to early or 

 prehistoric politics, and, indeed, to late politics, this you 

 know is your view. 



Letter 216 A. R. Wallace to C. Darwin. 



9, St. Mark's Crescent, N.W., August i6th [1868]. 



I ought to have written before to thank you for the 

 copies of your papers on Primula and on " Cross-unions of 

 Dimorphic Plants, etc." The latter is particularly interesting 

 and the conclusion most important ; but I think it makes 

 the difficulty of how these forms, with their varying degrees 

 of sterility, originated, greater than ever. If " natural selec- 

 tion " could not accumulate varying degrees of sterility for 

 the plant's benefit, then how did sterility ever come to be 

 associated with one cross of a trimorphic plant rather than 

 another ? The difficulty seems to be increased by the 

 consideration that the advantage of a cross with a distinct 

 individual is gained just as well by illegitimate as by 

 legitimate unions. By what means, then, did illegitimate 

 unions ever become sterile ? It would seem a far simpler 

 way for each plant's pollen to have acquired a prepotency 

 on another individual's stigma over that of the same 

 individual, without the extraordinary complication of three 

 differences of structure and eighteen different unions with 

 varying degrees of sterility ! 



However, the fact remains an excellent answer to the 

 statement that sterility of hybrids proves the absolute dis- 

 tinctness of the parents. 



I have been reading with great pleasure Mr. Bentham's 

 last admirable address, 2 in which he so well replies to the 



1 "Physic and Politics," Fortnightly Review, Vol. III., p. 452, 1868. 



2 Proc. Linn. Soc., 1867-8, p. Ivii. 



