i866 — 1S72] SEXUAL SELECTION 97 



of producing sterile workers ! But I have been most inter- L.-tte: 

 estcd by your discussion on secondary sexual differences ; I 

 do not suppose so full an account of such differences in any 

 other group of animals has ever been published. It delights 

 me to find that we have independently arrived at almost 

 exactly the same conclusion with respect to the more im- 

 portant points deserving investigation in relation to sexual 

 selection. For instance, the relative number of the two 

 sexes, the earlier emergence of the males, the Laws of in- 

 heritance, etc. What an admirable illustration you give of 

 the transference of characters acquired by one sex — namely, 

 that of the male of Bombus possessing the pollen-collecting 

 apparatus. Many of your facts about the differences between 

 male and female bees are surprisingly parallel with those 

 which occur with birds. The r your essay has given 



me great confidence in tin.' efficacy of sexual selection, and 

 1 wanted some encouragement, as extremely few naturalis: 

 in England seem inclined to believe in it. I am, however, 

 glad to find that Prof. Wiesmann has some faith in this 

 principle. 



The males of Bombus follow one remarkable habit, which I 

 think it would interest you to investigate this coming summer, 

 and no one could do it better than you. 1 I have therefore 

 enclosed a briefly and roughly drawn-up account of this habit. 

 Should you succeed in making any observations on this sub- 

 ject, and if you would like to use in any way my MS. you are 

 perfectly welcome. I could, should you hereafter wish to 

 make any use of the facts, give them in rather fuller detail ; 

 but I think that I have given enough. 



1 hope that you may long have health, leisure, and in- 

 clination to do much more work as excellent as your recent 

 essay. 



1 Mr. Darwin's observations on this curious subject were sent to 

 Hermann Muller, and after his death were translated and published in 

 Kxause's Gesammelte kleinere Schriften von Charles Darwin^ 18S7, p. 84. 

 The male bees had certain regular lines of flight at Down, as from t lie end ot" 

 the kitchen garden to the corner of the "sand-walk," and certain regular 

 "buzzing places'' where they stopped on the wing for a moment or two. 

 Mr. Darwin's children remember vividly the pleasure of helping in the 

 investigation of this habit. 



VOL. II. 7 



