176 THE EVOLUTIONIST AT LARGE. 



so continuous that one might pass over speci- 

 mens of three or four different kinds, arranged 

 in rows, without ever being able to say quite 

 clearly, by the eye alone, where one group 

 ended and the next group began. Among 

 the snails such an arrangement is peculiarly 

 easy ; for some of the species are very indefi- 

 nite, and the varieties are numerous under 

 each species. Nothing can give one so good 

 a notion of the plasticity of organic forms as 

 such a method. The endless varieties and 

 intermediate links which exist amongst dogs is 

 the nearest example to it with which ordinary- 

 observers are familiar. 



But the cyclostoma is a snail which intro- 

 duces one to still deeper questions. It 

 belongs in all our scientific classifications to 

 the group of lung-breathing mollusks, like 

 the common garden snail. Yet it has one 

 remarkable peculiarity : it possesses an oper- 

 culum, or door to its shell, like that of 

 the periwinkle. This operculum represents 

 among the univalves the under-shell of the 



