258 THE DESCENT OF MAN. 



namely the union of the two sexes in some of these animals, the 

 permanently affixed condition of others, and the low mental 

 powers of all, that such colors do not serve as a sexual attraction, 

 and have not been acquired through sexual selection. It should 

 be borne in mind that in no case have we sufficient evidence that 

 colors have been thus acquired, except where one sex is much 

 more brilliantly or conspicuously colored than the other, and 

 where there is no difference in habits between the sexes sufficient 

 to account for their different colors. But the evidence is rendered 

 as complete as it can ever be, only when the more ornamented 

 individuals, almost always the males, voluntarily display their 

 attractions before the other sex; for we cannot believe that such 

 display is useless, and if it be advantageous, sexual selection 

 will almost inevitably follow. We may, however, extend this 

 conclusion to both sexes, when colored alike, if their colors are 

 plainly analogous to those of one sex alone in certain other 

 species of the same group. 



How, then, are we to account for the beautiful or even gor- 

 geous colors of many animals in the lowest classes? It appears 

 doubtful whether such colors often serve as a protection; but 

 that we may easily err on this head, will be admitted by every 

 one whu reads Mr. Wallace's excellent essay on this subject. It 

 would not, for instance, at first occur to any one that the trans- 

 parency of the Medusae, or jelly-fishes, is of the highest service 

 to them as a protection; but when we are reminded by Hackel 

 that not only the medusae, but many floating mollusca, crusta- 

 ceans, and even small oceanic fishes partake of this same glass- 

 like appearance, often accompanied by prismatic colors, we can 

 hardly doubt that they thus escape the notice of pelagic birds 

 and other enemies. M. Giard is also convinced' that the bright 

 tints of certain sponges and ascidians serve as a protection. Con- 

 spicuous colors are likewise beneficial to many animals as a 

 warning to their would-be devourers that they are distasteful, or 

 that they possess some special means of defense; but this subject 

 will be discussed more conveniently hereafter. 



We can, in our ignorance of most of the lowest animals, only 

 say that their bright tints result either from the chemical na- 

 ture or the minute structure of their tissues, independently of any 

 benefit thus derived. Hardly any color is finer than that of ar- 

 terial blood; but there is no reason to suppose that the color of 

 the blood is in itself any advantage; and though it adds to the 

 beauty of the maiden's cheek, no one will pretend that it has been 

 acquired for this purpose. So again with many animals, espe- 

 cially the lower ones, the bile is richly colored; thus, as I am in- 

 formed by Mr. Hancock, the extreme beauty of the Eolidffi (naked 



» 'Archives de Zoolog-. Exper.,' Oct. 3872, p. 563. 



