SEXUAL SELECTION. 265 



in the active struggle for life whicli takes place on the 

 surface of the ocean. They were enabled to approach their 

 prey the most easily unobserved, and were themselves least 

 observed by their enemies. Hence they could preserve and 

 propagate themselves more easily than their more coloured 

 and opaque relatives; and finally, by accumulative adaptation 

 and transmission by inheritance, through natural selection, 

 in the course of many generations their bodies would attain 

 that degree of crystal-like transparency and colourlessness 

 which we at present admire in them. (Gen. Morph. ii. 242.) 

 No less interesting and instructive than homochromic 

 selection is that species of natural selection which Darwin 

 calls "sexual selection,'' which explains the origin of the 

 so-called " secondary sexual characters." We have already 

 mentioned these subordinate sexual characteristics, so in- 

 structive in many respects. They comprise those pecu- 

 liarities of animals and plants which belong only to one 

 of the two sexes, and which do not stand in any direct 

 relation to the act of propagation itself (compare above, 

 p. 244). Such secondary sexual characters occur in great 

 variety among animals. We all know how striking is the 

 difference of the two sexes in size and colour in many birds 

 and butterflies. The male sex is generally the larger and 

 more beautiful. It often possesses special decorations or 

 weapons ; as for example, the spur and comb of the cock, 

 the antlers of the stag and deer, etc. All these peculiarities 

 of the two sexes have nothing directly to do with pro- 

 pagation itself, which is effected by the "primary sexual 

 characters," or actual sexual organs. 



Now, the origin of these remarkable " secondary sexual 

 characters " is explained by Darwin simply by a choice or 



