SEXUAL SELECTION. 26 5 
in the active struggle for life which 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 
