29G THE DESCENT OF MAN. 



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 

 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, as 

 I am informed by Mr. Hancock, the extreme beauty cf the 

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

 being 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 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 thus gained, in the complex laboratory of living 

 organisms. 



The Sul) Kingdom of Hie Molluska. Throughout this 

 great division of the animal kingdom, as far as I can dis- 

 cover, secondary sexual characters, such as we are here con- 

 sidering, never occur. Nor could they be expected in the 

 three lowest classes, namely, in the Ascidiaiis, Polyzoa and 

 Brachiopods (constituting the Molluscoida of some authors), 

 for most of these animals are permanently affixed to a sup- 

 port or have their sexes united in the same individual. In 

 the Lamellibranchiata, or bivalve shells, hermaphroditism 

 is not rare. In the next higher class of the Gasteropoda, 

 or univalve shells, the sexes are either united or separate. 

 But in the latter case the males never possess special organs 

 for finding, securing, or charming the females, or for fight- 

 ing with other males. As I am informed by Mr. Gwyn 

 Jeffreys, the sole external difference between the sexes con- 

 sists in the shell sometimes diifering a little in form; for 

 instance, the shell of the male periwinkle (Littorina 

 lit tor ea) is narrower and has a more elongated spire than 

 that of the female. But differences of this nature, it may 



