LIFE ON THE EARTH. 109 



Postglacial shelly marls, &c. 

 Preglacial shelly marls, &c. 

 Csenozoic Period.. 



Alternating freshwater and marine 



Strata of the Isle of Wight. 



Mesozoic Period . . . The deposits of the Weald of Sussex. 



The deposits of Whitbyand Scarborough. 



Palaeozoic Period ... The Coal-formation. 



The Upper part of the Old Bed Sand- 

 stone of Ireland. 



And besides, there are several cases of the inter- 

 mixture of land-plants and insects and sea-shells, 

 only to be explained by the flowing of currents 

 from the land, as at Stonesfield, Westbury Cliff, &c. 



The forms of life in the Fresh waters of the Earth 

 differ from those in the Sea in all cases; but the 

 difference can seldom be traced to any physiological 

 necessity, arising from the difference of the fluids. 

 Amorphozoa, Zoophyta, Mollusca, Annelida, Crustacea, 

 Fishes, Reptiles and Mammalia, occur in both, under 

 shapes not indeed identical, but fashioned on the 

 same general models. The salmon migrates from 

 one water to the other, and experiments appear to 

 shew that by long continuance of favourable circum- 

 stances other fishes and some Mollusca might be 

 made to exchange elements, or to subsist for a time 

 in brackish water, a fluid of intermediate character. 

 But, on a large scale, the provisions of nature keep 



