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 great 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 
