270 THE SPREAD OF EVOLUTION". [Oh. XIV. 



black and red colours, whilst feeding on large green leaves. If 

 any one objected to male butterflies having been made beautiful 

 by sexual selection, and asked why should they not have been 

 made beautiful as well as their caterpillars, what would you 

 answer ? I could not answer, but should maintain my ground. 

 Will you think over this, and some time, either by letter or 

 when we meet, tell me what you think ? . . . . 



He seems to have received an explanation by return of post, 

 for a day or two afterwards he could write to Wallace : — 



" Bates was quite right ; you are the man to apply to in a 

 difficulty. I never heard anything more ingenious than your 

 suggestion, and I hope you may be able to prove it true. That 

 is a splendid fact about the white moths ; it warms one's very 

 blood to see a theory thus almost proved to be true." 



Mr. Wallace's suggestion was that conspicuous caterpillars 

 or perfect insects {e.g. white butterflies), which are distasteful 

 to birds, benefit by being promptly recognised and therefore 

 easily avoided.* 



The letter from Darwin to Wallace goes on : " The reason 

 of my being so much interested just at present about sexual 

 selection is, that I have almost resolved to publish a little essay 

 on the origin of Mankind, and I still strongly think (though 

 I failed to convince you, and this, to me, is the heaviest blow 

 possible) that sexual selection has been the main agent in 

 forming the races of man. 



" By the way, there is another subject which I shall introduce 

 in my essay, namely, expression of countenance. Now, do you 

 happen to know by any odd chance a very good-natured and 

 acute observer in the Malay Archipelago, who you think would 

 make a few easy observations for me on the expression of the 

 Malays when excited by various emotions ? " 



The reference to the subject of expression in the above 

 letter is explained by the fact, that my father's original inten- 

 tion was to give his essay on this subject as a chapter in the 

 Descent of Man, which in its turn grew, as we have seen, out of 

 a proposed chapter in Animals and Plants. 



He got much valuable help from Dr. Gunther, of the Natural 

 History Museum, to whom he wrote in May 1870 : — 



" As I crawl on with the successive classes I am astonished 

 to find how similar the rules are about the nuptial or ' wedding 



* Mr. Jeuner Weir's observations published in the Transactions of the 

 Entomological Society (1869 and 1870) give strong support to the theory 

 in question. 



