Alfred Russel Wallace 



of crossing^ and as regular^ I admit it would be a most 

 serious objection. But it is not. I believe many distinct 

 species can be grafted while others less distinct cannot. 

 The regularity with which natural species are sterile 

 together, even when very much alike ^ I think is an argu- 

 ment in favour of the sterility having been generally pro- 

 duced by Natural Selection for the good of the species. 



The other difficulty, of unequal sterility of reciprocal 

 crosses, seems none to me; for it is a step to more com- 

 plete sterility, and as such would be useful and would be 

 increased by selection. 



I have read Sir C. LyelPs second volume with great 

 pleasure. He is, as usual, very cautious, and hardly ever 

 expresses a positive opinion, but the general effect of the 

 whole book is very strong, as the argument is all on 

 our side. 



I am in hopes it will bring in a new set of converts to 

 Natural Selection, and will at all events lead to a fresh 

 ventilation of the subject. — Believe me yours very faith- 



^^^^J> Alfred E. Wallace. 



4 Chester Place, Regent's Park, N.W, March 27, 1868. 



My dear Wallace, — My son has failed in your problem, 

 and says that it is *^ excessively difficult '^ : he says you 

 will find something about it in Thomson and Tait, '^ Natural 

 Philosophy " (art. 649). He has, however, sent the solution, 

 if the plate rested on a square rim, but he supposes this 

 will not answer your purpose; nevertheless, I have for- 

 warded it by this same post. It seems that the rim being 

 round makes the problem much more difficult. 



I enclose my photograph, which I have received from 

 Down. I sent your answer to George on his objection to 

 your argument on sterility, but have not yet heard from 

 him. I dread beginning to think over this fearful prob- 



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