204 



EVOLUTION AND ANIMAL LIFE 



resulted in depression at one point and elevation at another, 

 land has become ocean and ocean land. And, in the aln ost 

 unimaginable period of time which has passed since the earth 

 first shrank from its hypothetical condition of nebulous vapor 

 to be a ball of land covered with water, such changes have 

 occurred over and over again. They have, however, mostly 

 taken place slowly and gradually. The principal seat of great 



Fig. 175. — Restoration of the skeleton in probable normal position of Dimorphodon 



macronyx. (After Seeley.) 



change is in the regions of mountain chains, which, in most 

 cases, are simply the remains of old folds or wrinkles in the 

 crust of the earth. 



When an aquatic animal dies, it sinks to the bottom of the 

 lake or ocean, unless, of coiu-se, its flesh is eaten by some other 

 animal. Even then its hard parts will probably find their 

 way to the bottom. There 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 obvious that aquatic animals are far more 

 Ukely to be preserved as fossils than land animals. This 



