314 SEXUAL SELECTION. [Part IL 



ed by Hackel that not only the medusae but many float- 

 ing mollusca, crustaceans, and even small oceanic fishes 

 partake of this same glass-like structure, we can hardly 

 doubt that they thus escape the notice of pelagic birds and 

 ether enemies. 



Notwithstanding our ignorance how far color in many 

 cases serves as a protection, the most probable view in re- 

 gard to the splendid tints of many of the lowest animals 

 seems to be that their colors are the direct result 'either 

 of the chemical nature or the minute structure of their 

 tissues, independently of any benefit thus derived. Hard- 

 ly any color is finer than that of arterial 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, 

 especially the lower ones, the bile is richly colored ; thus 

 the extreme beauty of the Eolidse (naked sea-slugs) is 

 chiefly due, as I am informed by Mr. Hancock, to the 

 biliary glands seen through the translucent integuments ; 

 this beauty being probably of no service to thes.e animals. 

 The tints of the decaying leaves in an American forest are 

 described by every one as gorgeous ; yet no one supposes 

 that these tints are of the least advantage to the trees. 

 Bearing in mind how many substances closely analogous 

 to natural organic compounds have been recently formed 

 by chemists, and which exhibit the most splendid colors, 

 it would have been a strange fact if substances similarly 

 colored had not often originated, independently of any 

 useful end being thus gained, in the complex laboratory 

 of living organisms. 



The sub-kingdom of the Mollusca. — Throughout this 

 great division (taken in its largest acceptation) of the 

 animal kingdom, secondary sexual characters, such as we 



