Alfred Russel Wallace 



parents originating in a distinct locality, and perhaps with a 

 very different constitution, merely because it also is sterile 

 with the bulk of the species, should be fertile with the very 

 same individuals, X, Y, Z, that A is fertile with. It seems 

 to me to be at least 100 to 1 that it will be fertile with some 

 other quite distinct set of individuals. And so with C, and 

 any other similar variety. I express this by saying that each 

 has its ^' sexual complements," and that the complements of 

 the one are almost sure not to be the complements of the 

 other. Hence it follows that A, B, C, though differing in the 

 same character of general infertility with the bulk of the 

 species, will really be three distinct varieties physiologically, 

 and can in no way unite to form a single physiological 

 variety. This enormous difficulty Romanes apparently never 

 sees, but argues as if all individuals that are infertile with 

 the bulk of the species must be or usually are fertile with the 

 same set of individuals or with each other. This I call a 

 monstrous assumption, for which not a particle of evidence 

 exists. Take this in conjunction with my argument from 

 the severity of the struggle for existence and the extreme 

 improbability of the respective ^' sexual complements " com- 

 ing together at the right time, and I think Romanes' 

 ponderous paper is disposed of. 



I wrote my paper, however, quite as much to expose the 

 great presumption and ignorance of Romanes in declaring 

 that Natural Selection is not a theory of the origin of species 

 — as it is calculated to do much harm. See, for instance, the 

 way the Duke of Argyll jumped at it like a trout at a fly ! — 

 Yours very faithfully, Alfred R. Wallace. 



The earlier part of the next letter refers to ^' The Experi- 

 mental Proof of the Protective Value of Colour and Markings 

 in Insects in reference to their Vertebrate Enemies," in the 

 Proceedings of the Zoological Society of London, 1887, p. 191. 



38 



