PALEONTOLOGY -117 



forms, while B is intermediate in structure between 

 A and C. Then the doctrine of evolution offers four 

 possible alternatives. A may have become C by way 

 of B : or C may have become A by way of B : or A 

 and C may be independent modifications of B: or A, 

 B and C may be independent modifications of some 

 unknown D." 



It is evident also that in order to favor the theory 

 of evolution fossils must show the proper chronolog- 

 ical arrangement, and that they must be located geo- 

 graphically in such a way that it would be possible to 

 evolve the one from the other. 



I will briefly consider the geological evidence as to 

 the evolution of vertebrates. 



The earliest known vertebrates were the fishes of 

 the Trenton period of the lower Silurian. Dana says 

 of them: "Remains of Fishes, the earliest known 

 Vertebrates, occur in the rocks of the Trenton 

 period. . . . The fossils are abundant near Canon 

 City, Col. Most of them are plates and scales of 

 Ganoids, the largest about half an inch across. Of 

 these plates, . . . two are referred to Placoderms, 

 the group which comprises the oldest Fishes pre- 

 viously known, those of the Upper Silurian and early 

 Devonian. The scales have the markings of a typical 

 Ganoid, much like those of the genus Holoptychius, a 

 form not found hitherto in beds earlier than the Mid- 

 dle Devonian. Besides these, there are remains of 

 what are supposed to be the ossified sheaths of a 

 notochord of a species of the Shark tribe related to 

 the Chimaera. The beds affording these remains of 

 Fishes contain many other fossils that are referred to 

 the Lower Trenton, and are overlaid by others carry- 

 ing Upper Trenton fossils." * 



Geikie says: "The first traces of vertebrate life 



* Dana's Manual of Geology, Fourth Edition, 1895, pp. 509-10. 



