106 ORGANIC EVOLUTION 



rocks of more recent formation. Take any group and com- 

 pare a number of Cambrian fossils of this group with a num- 

 ber from the younger rocks and we find the younger fossils 

 decidedly higher in their organization. In the rocks formed 

 during the Silurian age, which succeeded the Cambrian 

 period, we find the vertebrates, fishes, beginning to appear, 

 and the earliest air-breathing animals, insects and scorpions, 

 also animals of the same groups that we found represented 

 in the Cambrian rocks, but of a more elaborate structure. 

 In the Devonian period cartilaginous and ganoid fishes and 

 terrestrial and fresh-water shells are among the most inter- 

 esting forms. In the next younger rocks, the Carboniferous, 

 appear the earliest Amphibia as well as more highly organ- 

 ized representatives of the several groups of invertebrates. 

 The earliest reptiles appear in the Permian rocks, which 

 follow the Carboniferous. Mammals and birds are found 

 in the rocks of the succeeding two periods, and all of the 

 groups of vertebrates and invertebrates continue to be repre- 

 sented by progressively more highly differentiated species, 

 many of the more lowly types disappearing, until we come 

 to the present age, commonly called the age of man. This 

 general sequence of fossils, the simpler giving way to the 

 more complex as we come down to the younger rocks, is 

 a most impressive thing, and is one of the chief evidences 

 that evolution has taken place. 



Turning to a few illustrations of the origin of particu- 

 lar species or organs, we find the same principle of grad- 

 ual increase in complexity as we come from the older to 

 the younger geological formations. Our record of the evo- 

 lution of branching antlers in the deer is fairly complete 

 (Fig. 25). The first deer in the early Miocene had no 



