SLUGS AND SNAILS. 55 



have taken to crawling up the leaves of 

 marsh-plants, and have thus gradually accli- 

 matised themselves to a terrestrial existence. 

 We can trace a perfectly regular series from 

 the most aquatic to the most land-loving 

 species, just as I have tried to trace a regular 

 series from the shell-bearing snails to the 

 shell-less slugs. Well, when the earliest 

 common ancestor of both these last-named 

 races first took to living above water, he 

 possessed a horny shell (like that of the 

 amber-snail), which his progenitors used to 

 manufacture from the mineral matters dis- 

 solved in their native streams. Some of the 

 younger branches descended from this pri- 

 maeval land-snail took to living on very dry 

 land, and when they reached chalky districts 

 manufactured their shells, on an easy and 

 improved principle, almost entirely out of 

 lime. But others took to living in moist 

 and boggy places, where mineral matter was 

 rare, and where the soil consisted for the 

 most part of decaying vegetable mould. 



