20 ANIMALS OF THE PAST 



scent obscure, and how soon gaps appear whose 

 filhng in requires much patient research. How 

 much more difficult must it be, then, to trace 

 the pedigree of a race that extends, not over 

 centuries, but thousands of centuries ; how wide 

 must be some of the gaps, how very different 

 may the founders of the family be from their 

 descendants ! The words old and ancient that 

 we use so often in speaking of fossils appeal to 

 us somewhat vaguely, for we speak of the an- 

 cient civilizations of Greece and Rome, and call 

 a family old that can show a pedigree running 

 back four or five hundred years, when such as 

 these are but affairs of yesterday compared 

 with even recent fossils. 



Perhaps we may better appreciate the mean- 

 ing of these words by recaUing that, since the 

 dawn of vertebrate life, sufficient of the earth's 

 surface has been worn away and washed into 

 the sea to form, were the strata piled directly 

 one upon the other, fifteen or twenty miles of 

 rock. This, of course, is the sum total of sedi- 

 mentary rocks, for such a thickness as this is not 

 to be found at any one locality; because, during 

 the various ups and downs that this world of 



