140 THE VALUE OF COLOUR 



generally decided by the law of battle, but in the case of 

 birds, apparently, by the charms of their song, by their 

 beauty or their power of courtship, as in the dancing rock- 

 thrush of Guiana. The most vigorous and healthy males, 

 implying perfect adaptation, must generally gain the victory 

 in their contests. This kind of selection, however, is less 

 rigorous than the other ; it does not require the death of 

 the less successful, but gives to them fewer descendants. 

 The struggle falls, moreover, at a time of year when food is 

 generally abundant, and perhaps the effect chiefly produced 

 would be the modification of the secondary sexual characters, 

 which are not related to the power of obtaining food, or to 

 defence from enemies, but to fighting with or rivalling 

 other males. The result of this struggle amongst the males 

 may be compared in some respects to that produced by those 

 agriculturists who pay less attention to the careful selection 

 of all their young animals, and more to the occasional use of 

 a choice mate.' 



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.' l 



Darwin, as we know from his Autobiography, 2 

 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 



1 More Letters, ii. 33. 2 Life and Letters, i. 94. 



