THE ANIMALS OF THE PAST 423 



Even then its hard parts will probably find their way to the 

 bottom. At the bottom the remains will soon be covered 

 by the always dropping sediment. They are on the way to 

 become fossils. Some land animals also might, after 

 death, get carried by a river to the lake or ocean, and find 

 their way to the bottom, where they, too, will become 

 fossils. Or they may die on the banks of the lake or ocean 

 and' their bodies may get buried in the soft mud of the 

 shores. Or, again, they are often trodden in the mire 

 about salt springs or submerged in quicksands. It is ob- 

 vious that aquatic animals are far more likely to be pre- 

 served as fossils than land animals. This inference is 

 strikingly proved by fossil remains. Of all the thousands 

 and thousands of kinds of extinct insects, mostly land 

 animals, comparatively few 

 specimens are known as fos- 

 sils. On the other hand, 

 the shell-bearing mollusks 

 and crustaceans are repre- 

 sented in almost all rock 

 deposits which contain any 

 kind of fossil remains. 



It is obvious that any por- 

 tion Of the earth's Surface FIG. 244. -A fossil brachiopod (Spirlfer 



covered by stratified rocks TIlT^T^' ' 

 must have been at some time 



under water, the bottom of a lake or ocean. If now this 

 portion shows a series of layers or strata of different kinds 

 of sedimentary rocks, it is evident that it must have been 

 under water several times, or at least under different con- 

 ditions. It is also evident that fossils found in this portion 

 of the earth will contain remains of only those animals 

 which were living at the various times this portion of the 

 earth was under water. Of the animals which lived on it 

 when it was land, there will be no trace, except, possibly, a 

 few land or fresh-water forms which might be swept into the 



