262 The Descent of Man. Part U 



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 colours, it would have been a strange fact if substances 

 similarly coloured had not often originated, independently of 

 any useful end thus gained, in the complex laboratory of living 



organisms. 



The sub-kingdom of the Mollusca. — Throughout this great 

 division of the animal kingdom, as far as I can discover, 

 secondary sexual characters, such as we are here considering, 

 never occur. Nor could they be expected in the three lowest 

 classes, namely in the Ascidians, Polyzoa, and Brachiopods 

 (constituting the Molluscoida of some authors), for most of 

 these animals are permanently affixed to a support 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 fighting with other males. As I am informed 

 by Mr. Gwyn Jeffreys, the sole external difference between the 

 sexes consists in the shell sometimes differing a little in form ; 

 for instance, the shell of the male periwinkle (Littorina littorea) 

 is narrower and has a more elongated spire than that of the 

 female. But differences of this nature, it may be presumed, are 

 directly connected with the act of reproduction, or with the 

 development of the ova. 



The Gasteropoda, though capable of locomotion and furnished 

 with imperfect eyes, do not appear to be endowed with sufficient 

 mental powers for the members of the same sex to struggle 

 together in rivalry, and thus to acquire secondary sexual 

 characters. Nevertheless with the pulmoniferous gasteropods, or 

 land-snails, the pairing is preceded by courtship; for these 

 animals, though hermaphrodites, are compelled by their structure 

 to pair together. Agassiz remarks, 2 " Quiconque a eu l'occasion 

 " d'observer les amours des limacons, ne saurait mettre en doute 

 11 la seduction deployee dans les mouvements et les allures qui 

 t€ preparent et accomplissent le double embrassement de ces 

 " hermaphrodites." These animals appear also susceptible of 

 some degree of permanent attachment : an accurate observe l*, 

 2 ' De 1'Espece tt de la Class. &c, 1869, p. 106. 



