324 SEXUAL SELECTION. Part II. 



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 are here considering, never, as far as I can discover, 

 occur. Nor could they be expected in the three lowest 

 classes, namely in the Ascidians, Polyzoa, and Brachio- 

 pods (constituting the Molluscoida of Huxley), 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, hermaphro- 

 ditism is not rare. In the next higher class of the 

 Gasteropoda, or marine univalve shells, the sexes are 

 either united or separate. But in this latter case the 

 males never possess special organs for finding, securing, 

 or charming: the females, or for fio-htino- with other 

 males. The sole external difference between the sexes 

 consists, as I am informed by Mr. Gwyn Jeffreys, 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 en- 

 dowed with sufficient mental powers for the members 

 of the same sex to struggle together in rivalry, and 

 thus to acquire secondary sexual characters. Never- 

 theless 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, 1 " Qui- 

 " conque a eu l'occasion d'observer les amours cles lima- 



1 ' De l'Espece et cle la Class.' &c, 1869, p. 106. 



