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 myriads of creatures may have flour- 
ished and passed away without so much as 
leaving a trace of their presence behind. 
