1869.] DR. H. THIEL. 293 



low in the scale sexual differences occur which require some 

 degree of self-consciousness in the males, as weapons by 

 which they fight for the female, or ornaments which attract 

 the opposite sex. Any differences between males and females 

 which follow different habits of life would have to be ex- 

 cluded. I think you will easily see what I wish to learn. A 

 priori^ it would never have been anticipated that insects 

 would have been attracted by the beautiful colouring of the 

 opposite sex, or by the sounds emitted by the various musical 

 instruments of the male Orthoptera. I know no one so likely 

 to answer this question as yourself, and should be grateful for 

 any information, however small. 



My second subject refers to expression of countenance, to 

 which I have long attended, and on which I feel a keen in- 

 terest ; but to which, unfortunately, I did not attend when I 

 had the opportunity of observing various races of man. It 

 has occurred to me that you might, without much trouble, 

 make a few observations for me, in the course of some 

 months, on Negroes, or possibly on native South Americans, 

 though I care most about Negroes ; accordingly I enclose 

 some questions as a guide, and if you could answer me even 

 one or two I should feel truly obliged. I am thinking of 

 writing a little essay on the Origin of Mankind, as I have 

 been taunted v/ith concealing my opinions, and I should do 

 this immediately after the completion of my present book. 

 In this case I should add a chapter on the cause or meaning 

 of expression. ... 



[The remaining letters of this year deal chiefly with the 

 books, reviews, &c., which interested him.] 



C. Darwin to H. Thiel. 



Down, February 25 1869. 



Dear Sir, — On my return home after a short absence, I 

 found your very courteous note, and the pamphlet,* and I 



* ' Ueber einige Formen der Landwirtlischaftlichen Genossenschaften.' 

 By Dr. H. Thiel, then of the Agricultural Station at Poppelsdorf. 



