12 ANIMALS OF THE PAST 



so that vertebrates are considered by some to have 

 descended from the worms, while others have found 

 their beginnings in some animal allied to the King 

 Crab. 



Every student of genealogy knows only too well how 

 difficult a matter it is to trace a family pedigree back a 

 few centuries, how soon the family names become 

 changed, the line of descent obscure, and how soon 

 gaps appear whose filling in requires much patient re- 

 search. How much more difficult must it be, then, to 

 trace the pedigree of a race that extends, not over cen- 

 turies, 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 

 ancient 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 meaning of 

 these words by recalling that, since the dawn of verte- 

 brate 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 sedimen- 

 tary 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 ours has met with, 

 those portions that chanced to be out of water would re- 

 ceive no deposit of mud or sand, and hence bear no 

 corresponding stratum of rock. The reader may think 

 that there is a great deal of difference between fifteen 

 and twenty miles, but this liberal margin is due to the 

 difficulty of measuring the thickness of the rocks, and in 



