HOW IMBEDDED. 113 



and occurrence of fossils will be best explained by an 

 appeal to the existing operations of nature. If we stand by 

 the side of a river, and especially when it is in flood, we 

 perceive that the current is continually bearing onward 

 vegetable and animal debris, and that this debris is gradually 

 entombed among the mud, sand, and gravel which the river 

 deposits in the lake, estuary, or sea into which it discharges 

 its waters. As with one river, so with every rill and river 

 that traverses the terrestrial surface each is carrying 

 down the spoils of the land in a state more or less frag- 

 mentary, and burying them in the silt, where, excluded 

 from atmospheric decay, they appear in the first stages of 

 fossilisation. As with land plants and animals, so with 

 those of lakes and seas j they die and are imbedded in sedi- 

 ments where they lived and grew, or are drifted by tides 

 and currents to some distant locality. This process is ever 

 going forward in every region tropical, temperate, and 

 arctic; and as each region is characterised by its own 

 special flora and fauna, the imbedded remains will indicate 

 to future observers the external conditions under which they 

 grew and were deposited. The plants and animals entombed 

 in the estuary of the Amazon must differ from those de- 

 posited in the delta of the Mississippi, and these again from 

 those preserved in the mud-islands of the Niger, the Ganges, 

 and other Old World rivers. The shells, Crustacea, and 

 fishes that die and sink in the sediments of tropical seas 

 differ widely from those of temperate waters, and these 

 again as widely from the fauna of the Arctic and Ant- 

 arctic Oceans. As it is now, so it must have been in all 

 former ages, and thus the fossils of the stratified formations 

 become the only clue to the geographical conditions of the 

 areas in which they were deposited, and of the regions from 

 which they were derived. 



Nor is it geographical or climatic condition alone of which 



H 



