68 PAUT HISrOR Y OF ANIMALS. 



remains, how many will be destroyed by other living creat- 

 ures of various kinds, or obliterated by other destroymg 

 agents,— the air, and water, and acids. If we thmk, we 

 shall see that only bones, or fragments of bone, and espe- 

 cially teeth which are so hard, will be preserved upon the 

 land. But what are the rocks in which we seek for remams 

 of former life? They are the hardened, glued-together 

 fragments of pre-existing lands, the rocks of which, slowly 

 worn away by the ceaseless movements of water, have 

 been carried particle by particle by the rivers to the sea or 

 to some great lake, there to lie until some earth-movement 

 raised them above the surface of the waters, only for the 

 wearing away to begin once more. Thus we see how small 

 is the chance of any trace of animals that lived upon the 

 land being preserved ; indeed, we may say that only those 

 that happened to be drowned, or to fall when dead into 

 some lake, sea, or river, have other than the remotest chance 

 of leaving a trace. And further, we may remark how small is 

 the chance that such remains will be preserved in that 

 particular place, where, by reason of a cliff, a quarry, or a 

 cutting, we are able to look for them. It is only those 

 animals, then, that lived in water, that would be preserved 

 in any number. These dying would become covered up by 

 the constantly growing deposits of sand and mud brought 

 down by the rivers, or if in the deep sea, by the slow growth 

 of the ooze formed from the remains of the countless tiny 

 creatures that inhabit the surface waters of the ocean. 



But of aquatic animals we might expect a fair collection 

 of the extinct forms ; yet again we must remember, 



(i) That only their hard parts, bones, teeth, and shells, 

 are likely to be preserved to form " fossils " : 



(2) That of those that have been preserved, only a very 

 small percentage can ever be discovered, since it is only 

 where the rock is exposed that we can get at them : 



(3) That many vast areas of land have as yet not been 

 even seen by any one, while in others no thorough search 

 for fossils has been made : 



(4) That vast areas of rock are at present below the level 

 of the sea : 



(5) That many series of rocks that once existed, have 

 been entirely destroyed by subsequent denudation : 



