Ohap. IX. Sexual Selection. 26 1 



[t should be borne in mind that in no case have we suffi- 

 cient evidence that colours have been thus acquired, ex- 

 cept where one sex is much more brilliantly or conspicuously 

 coloured than the other, and where there is no difference 

 in habits between the sexes sufficient to account for their 

 different colours. But the evidence is rendered as complete 

 as it can ever be, only when the more ornamented indivi- 

 duals, 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 coloured alike, if their cc lours 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 

 gorgeous colours of many animals in the lowest classes? It 

 appears doubtful whether such colours often serve as a protec- 

 tion ; but that we may easily err on this head, will be admitted 

 by every one who reads Mr. Wallace's excellent essay on this 

 subject. It would not, for instance, at first occur to any one 

 that the transparency 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, crustaceans, and even small oceanic fishes 

 partake of this same glass-like appearance, often accompanied 

 by prismatic colours, we can hardly doubt that they thus 

 escape the notice of pelagic birds and other enemies. M. 

 Giarcl is also convinced 1 that the bright tints of certain 

 sponges and ascidians serve as a protection. Conspicuous 

 colours 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 defence; 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 

 nature or the minute structure of their tissues, independently of 

 any benefit thus derived. Hardly any colour is finer than that 

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

 colour 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 

 auimals, especially the lower ones, the bile is richly coloured ; 

 thus, as I am informed by Mr. Hancock, the extreme beauty of 

 the Eolidae (naked sea-slugs) is chiefly due to the biliary glands 

 1 'Archives de Zoolog. Exper.,' Oct. 1872, p. 563. 



