THE EVIDENCE FOR ORGANIC EVOLUTION 



511 



relationships can usually be determined by sufficient study; even 

 though strata that were originally beneath, and hence older, may 

 lie above the later deposits. The oldest fossils are, therefore, 

 in the lowest rocks, or in rocks that were the lowest in the undis- 

 turbed formation, for the same reason that the bottom layers 

 of a cut through filled ground are older than the upper ones. 

 By applying this simple principle to the rock outcrops in all parts 

 of the world, and with the aid of fossils, geologists have pieced 



Fig. 273. — Outcrop of sedimentary rock in horizontal strata, Bennett quar- 

 ries, North Buffalo,* N. Y. 



(At top is limestone, Onondaga, resting unconformably at a a upon an upper Silurian for- 

 mation, Cobleskill, which lies conformably along the line b b upon the cement rock, Bertie, 

 that reaches to the floor of the quarry. Photo, by courtesy of Professor Charles Schuchert.) 



together a series of sedimentary formations resting upon the earlier 

 igneous rocks and constituting the record of geologic history {cf. 

 Fig. 259). The fossils in these strata indicate the order of appear- 

 ance or succession of the various forms of Ufe. 



In the record as it appears, there is a succession from simpler 

 to more specialized types, which is of the greatest significance for 

 organic evolution. Not only are the animals of the past different 

 from those of the present, but the record begins with forms that 

 are vastly different. These are gradually succeeded by others 

 that become more like existing species until they merge with 



