296 Colour and the Struggle for Infe 
A full exposition of Sexual Selection appeared in The Descent of 
Man in 1871, and in the greatly augmented second edition, in 1874, 
It has been remarked that the two subjects, The Descent of Man and 
Selection in Relation to Sex, seem to fuse somewhat imperfectly 
into the single work of which they form the title. The reason for 
their association is clearly shown in a letter to Wallace, dated May 
28, 1864: “...I suspect that a sort of sexual selection has been the 
most powerful means of changing the races of man’.” 
Darwin, as we know from his Autobiography’, was always greatly 
interested in this hypothesis, and it has been shown in the preceding 
pages that he was inclined to look favourably upon it as an interpre- 
tation of many appearances usually explained by Natural Selection. 
Hence Sexual Selection, incidentally discussed in other sections of 
the present essay, need not be considered at any length, in the section 
specially allotted to it. 
Although so interested in the subject and notwithstanding his 
conviction that the hypothesis was sound, Darwin was quite aware 
that it was probably the most vulnerable part of the Origin. Thus 
he wrote to H. W. Bates, April 4, 1861: “If I had to cut up myself in 
a review I would have [worried?] and quizzed sexual selection; there- 
fore, though I am fully convinced that it is largely true, you may 
imagine how pleased I am at what you say on your belief®.” 
The existence of sound-producing organs in the males of insects 
was, Darwin considered, the strongest evidence in favour of the 
operation of sexual selection in this group*. Such a conclusion has 
received strong support in recent years by the numerous careful 
observations of Dr F. A. Dixey® and Dr G. B. Longstaff® on the 
scents of male butterflies. The experience of these naturalists 
abundantly confirms and extends the account given by Fritz Miiller’ 
of the scents of certain Brazilian butterflies. It is a remarkable fact 
that the apparently epigamic scents of male butterflies should be 
pleasing to man while the apparently aposematic scents in both sexes 
of species with warning colours should be displeasing to him. But 
the former is far more surprising than the latter. It is not perhaps 
astonishing that a scent which is ex hypothesi unpleasant to an 
insect-eating Vertebrate should be displeasing to the human sense ; 
but it is certainly wonderful that an odour which is ex hypothesi 
agreeable to a female butterfly should also be agreeable to man. 
1 More Letters, 11. p. 33. 2 Life and Letters, 1. p. 94. 
3 More Letters, 1. p. 183, 4 Life and Letters, 111, pp. 94, 138. 
5 Proc, Ent. Soc. Lond, 1904, p. lvi; 1905, pp. xxxvii, liv; 1906, p. ii. 
6 Proc. Ent. Soc. Lond, 1905, p. xxxv; Trans. Ent. Soc. Lond. 1905, p. 136; 1908, 
p- 607. 
7 Jen. Zeit. Vol. x1. 1877, p. 99; Trans. Ent. Soc. Lond. 1878, p, 211. 
