Chap. IX. SEXUAL SELECTION. 323 



protection ; but when we are reminded by Hackel that 

 not only the medusas but many floating mollusca, crus- 

 taceans, 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 other 

 enemies. 



Notwithstanding our ignorance how far colour in 

 many cases serves as a protection, the most probable 

 view in regard to the splendid tints of many of the 

 lowest animals seems to be that their colours are the 

 direct result either of 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 pur- 

 pose. So again with many animals, especially the lower 

 ones, the bile is richly coloured ; thus the extreme 

 beauty of the Eolidae (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 these animals. The 

 tints of the decaying leaves in an American forest are 

 described by every one as gorgeous ; yet no one sup- 

 poses 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 colours, it would have been a strange 

 fact if substances similarly coloured had not often 

 originated, independently of any useful end being 

 thus gained, in the complex laboratory of living 

 organisms. 



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