THE EARLIEST KNOWN VERTEBRATES 21 



ours has met with, those portions that chanced 

 to be out of water would receive 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 fif- 

 teen and twenty miles, but this liberal margin 

 is due to the difficulty of measuring the thick- 

 ness of the rocks, and in Europe the sum of 

 the measurable strata is much greater than in 

 North America. 



The earliest traces of animal life are found 

 deeper still, beneath something like eighteen 

 to twenty-five miles of rock, while below this 

 level are the strata in which dwelt the earliest 

 living things, organisms so small and simple 

 that no trace of their existence has been left, 

 and we infer that they were there because any 

 given group starts in a modest way with small 

 and simple individuals. 



At the bottom, then, of twenty miles of rocks 

 the seeker for the progenitor of the gi-eat fam- 

 ily of backboned animals finds the scant re- 

 mains of fish-like animals that the cautious 

 naturalist, who is much given to " hedging," 

 terms, not vertebrates, but prevertebrates or 



