154 THE METHOD OF DARWIN. 



males, and early in the species of which both 

 sexes have horns. He found that in seven 

 species with horns only on the males the horns 

 appear late, at nine months, or even later. 

 In the reindeer, which alone has horns on both 

 sexes, the horns appear in both sexes at the age 

 of four or five weeks. His investigations took 

 a somewhat wider range, and he gave other 

 illustrations and some exceptions to the law 

 among horned animals. 



One of the most striking of his minor efforts 

 was his explanation of the origin of the remark- 

 able color patterns called " ocelli " on the tail 

 of the peacock. 1 He had adopted the theory 

 of sexual selection for the explanation of the 

 beauty of birds, etc., which did not seem to 

 him to admit of explanation under natural selec- 

 tion. There are other naturalists who do not 

 believe that sexual selection plays the part in 

 nature which Darwin ascribed to it. But what- 

 ever view is taken of it, it furnished him with 

 the indispensable working hypothesis by means 

 of which to explain known facts and search for 

 others that ought to follow as consequences of 

 the hypothesis. It was characteristic of Dar- 

 win to select for investigation and explanation 

 extreme instances that would put his beliefs to 



1 Descent of Man, etc., Vol II. pp. 132-145. 



