16 ANIMALS OF THE PAST 



less in keeping the records — preserving them 

 mostly in scattered fragments — but after they 

 have been laid away and sealed up in the rocks 

 they are subject to many accidents. Some 

 specimens get badly flattened by the weight 

 of subsequently deposited strata, others are 

 cracked and twisted by the movements of the 

 rocks during periods of upheaval or subsidence, 

 and when at last they are brought to the sur- 

 face, the same sun and rain, snow and frost, 

 from which they once escaped, are ready to 

 renew the attack and crumble even the hard 

 stone to fragments. Such, very briefly, are 

 some of the methods by which fossils may be 

 formed, such are some of the accidents by 

 which they may be destroyed ; but this descrip- 

 tion must be taken as a mere outline and as 

 applying mainly to vertebrates, or backboned 

 animals, since it is with them that we shall have 

 to deal. It may, however, show why it is that 

 fossils are not more plentiful, why we have 

 mere hints of the existence of many animals, 

 and why myi'iads of creatures may have flour- 

 ished and passed away without so much as 

 leaving a trace of their presence behind. 



