90 MAN [Chap. VIII 



Letter 453 thought that the 13-year and the 17-year forms ought 

 not to be ranked as distinct species, unless other differences 

 besides the period of development could be discovered. They 

 thought the mere rarity of variability in such a point was 

 not sufficient, and I think I concur with them. The fact of 

 both the forms presenting the same case of dimorphism is 

 very curious. I have long wished that some one would 

 dissect the forms of the male stag-beetle with smaller 

 mandibles, and see if they were well developed, i.e., whether 

 there was an abundance of spermatozoa ; and the same 

 observations ought, I think, to be made on the rarer form 

 of your Cicada. Could you not get some observer, such as 

 Dr. Hartman, 1 to note whether the females flocked in equal 

 numbers to the " drumming " of the rarer form as to the 

 common form ? You have a very curious and perplexing 

 subject of investigation, and I wish you success in your 

 work. 



Letter 454 To A. R. Wallace. 



Down, June 15th [1869?]. 

 You must not suppose from my delay that I have not 

 been much interested by your long letter. I write now 

 merely to thank you, and just to say that probably you 

 are right on all the points you touch on, except, as I think, 

 about sexual selection, which I will not give up. My belief 

 in it, however, is contingent on my general belief in sexual 

 selection. It is an awful stretcher to believe that a peacock's 

 tail was thus formed ; but, believing it, I believe in the same 

 principle somewhat modified applied to man. 



Letter 455 To G. K. H. Thwaites. 2 



Down, February 13th [n.d.]. 



I wrote a little time ago asking you an odd question 

 about elephants, and now I am going to ask you an odder. 



1 Mr. Walsh sent Mr. Darwin an extract from Dr. Hartman's 

 " Journal of the doings of a Cicada septe?idecim" in which the females 

 are described as flocking round the drumming males. Descent of Man 

 (1901), p. 433. 



2 Dr. G. H. K. Thwaites (181 1-82) held for some years the post of 

 Director of the Botanic Gardens at Peradenyia, Ceylon ; and in 1864 

 published an important work on the flora of the island, entitled 

 Enumeratio Plantarum Zeylania:. 



