XXXVl INTRODUCTION. 



track them as far back in time as we can our marine species, 

 have, nevertheless, survived in many instances })roclig-ious 

 changes, and been the companions of lax-ger animals that 

 liave long ceased to inhabit our earth. Some of our com- 

 monest snails, such as Helix nemoralis, H. hispida, Zonites 

 cellaria, and Zua luhica, occur abundantly in deposits 

 buried beneath the gravel, in which the remains of extinct 

 elephants, and other quadrupeds, strikingly different from 

 the wild beasts now living in Central Europe, are abund- 

 antly found. Still older, possibly, are those that are asso- 

 ciated with the Cyrena of the Nile, and the Unio Uttoralis 

 in the fresh-water deposits of the valley of the Thames. 

 Geology has, as yet, scarcely approached towards an esti- 

 mate of the vast duration of these comparatively modern 

 epochs. They have but lately received anything like a due 

 share of attention, and the nearer we approach our own 

 times in our gropiugs amid geological antiquity, the more 

 complex seem to become our calculations of the length of 

 those periods during which the progenitors of still-existing 

 forms of life flourished, along with creatures that have dis- 

 appeared for ever. When we learn how long has been the 

 existence upon the world's surface of some of our little 

 shell-fish, our wonder is not that they should be so widely 

 spread over it now, but rather that there should be any 

 spot capable of supporting their life from which they should 

 be absent. 



