54 THE EVOLUTIONIST AT LARGE. 



of them require more or less lime or other 

 mineral matters, though in varying propor- 

 tions. Now, the snails which thrive best on 

 the bare chalk downs behind my little combe 

 belong to that pretty banded black-and-white 

 sort which everybody must have noticed 

 feeding in abundance on all chalk soils. In- 

 deed, Sussex farmers will tell you that South 

 Down mutton owes its excellence to these 

 fat little mollusks, not to the scanty herb- 

 age of their thin pasture-lands. The pretty 

 banded shells in question are almost wholly 

 composed of lime, which the snails can, of 

 course, obtain in any required quantity from 

 the chalk. In most limestone districts you 

 will similarly find that snails with calcareous 

 shells predominate. But if you go into a 

 granite or sandstone tract you will see that 

 horny shells have it all their own way. Now, 

 some snails with such houses took to living 

 in very damp and marshy places, which they 

 were naturally apt to do as indeed the land- 

 snails in a body are merely pond-snails which 



