288 EVOLUTION [Chap. IV 



including numerous species, a series can be formed from species which 

 when crossed yield fewer and fewer seeds, to species which never produce 

 a single seed, but yet are affected by the pollen of certain other species, 

 for the germen swells. It is here manifestly impossible to select the 

 more sterile individuals, which have already ceased to yield seeds ; so 

 that this acme of sterility, when the germen alone is affected, cannot 

 have been gained through selection ; and from the laws governing the 

 various grades of sterility being so uniform throughout the animal and 

 vegetable kingdoms, we may infer that the cause, whatever it may be, is 

 the same or nearly the same in all cases." 



Mr. Wallace, on the other hand, still adheres to his view : see his 

 Darwinism, 1889, p. 174, and for a more recent statement see p. 292, 

 note 1, Letter 211, and p. 299. 



The discussion of 1868 began with a letter from Mr. Wallace, written 

 towards the end of February, giving his opinion on the Variation of 

 Animals and Plants ; the discussion on the sterility of hybrids is at 

 p. 185, Vol. II. of the first edition. 



Letler 2 °9 A. R. Wallace to C. Darwin. 



Feb. 1868. 



The only parts I have yet met with where I somewhat 

 differ from your views, are in the chapter on the causes of 

 variability, in which I think several of your arguments are 

 unsound : but this is too long a subject to go into now. 

 Also, I do not see your objection to sterility between allied 

 species having been aided by Natural Selection. It appears 

 to me that, given a differentiation of a species into two forms, 

 each of which was adapted to a special sphere of existence, 

 every slight degree of sterility would be a positive advantage, 

 not to the individuals who were sterile, but to each form. If 

 you work it out, and suppose the two incipient species a . . . b 

 to be divided into two groups, one of which contains those 

 which are fertile when the two are crossed, the other being 

 slightly sterile, you will find that the latter will certainly 

 supplant the former in the struggle for existence ; remem- 

 bering that you have shown that in such a cross the 

 offspring would be more vigorous than the pure breed, and 

 therefore would certainly soon supplant them, and as these 

 would not be so well adapted to any special sphere of 

 existence as the pure species a and b, they would certainly in 

 their turn give way to a and b. 



