20 ANIMALS OF THE PAST 
scent obscure, and how soon gaps appear whose 
filling 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 recalling 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 
