1866-1872] SEXUAL SELECTION 67 



male and female birds ; for I do not understand that you Letter 435 

 yourself assert that the beak of the male is sensibly longer 

 than that of the female. If you come across any acute bird- 

 catchers (I do not mean to ask you to go after them), I wish 

 you would ask what is their impression on the relative 

 numbers of the sexes of any birds which they habitually 

 catch, and whether some years males are more numerous and 

 some years females. I see that I must trust to analogy (an 

 unsafe support) for sexual selection in regard to colour in 

 butterflies. You speak of the brimstone butterfly and genus 

 Edusa l (I forget what this is, and have no books here, unless 

 it is Colias} not opening their wings. In one of my notes to 

 Mr. Stainton I asked him (but he could or did not answer) 

 whether butterflies such as the Fritillaries, with wings bright 

 beneath and above, opened and shut their wings more than 

 Vanessce, most of which, I think, are obscure on the under 

 surface. That is a most curious observation about the red 

 underwing moth and the robin, 2 and strongly supports a 

 suggestion (which I thought hardly credible) of A. R. Wallace, 

 viz. that the immense wings of some exotic lepidoptera served 

 as a protection from difficulty of birds seizing them. I will 

 probably quote your case. 



No doubt Dr. Hooker collected the Kerguelen moth, for 

 I remember he told me of the case when I suggested in the 

 Origin, the explanation of the coleoptera of Madeira being 

 apterous ; but he did not know what had become of the 

 specimens. 



I am quite delighted to hear that you are observing 

 coloured birds, 3 though the probability, I suppose, will be 

 that no sure result will be gained. I am accustomed with 

 my numerous experiments with plants to be well satisfied if 

 I get any good result in one case out of five. 



You will not be able to read all my book too much 

 detail. Some of the chapters in the second volume are 



1 Colias Edusa. 



3 Descent of Man, Ed. I., Vol. I., p. 395. Mr. Weir describes the pur- 

 suit of a red-underwing, Triph&a pronuba, by a robin which was attracted 

 by the bright colour of the moth, and constantly missed the insect by 

 breaking pieces off the wing instead of seizing the body. Mr. Wallace's 

 facts are given on the same page. 



3 Descent of Man, Ed. I., Vol. II., p. no. 



