2Q2 



AN INTRODUCTION TO ZOOLOGY 



(c) Paleontology The study of fossil animals has brought 

 out some remarkably convincing evidence of organic evolution. 



FIG. 158. Skeletons of the fore limbs of various vertebrates, a, wing of 

 bird; b, fore leg of dog ; c, arm of man ; d, wing of bat. (From Met- 

 calf.) 



Table XV on page 280 shows that in the oldest strata of the earth's 

 crust containing organic remains, the Cambrian and Silurian, 

 only invertebrates existed. Toward the end of the latter period 

 fishes made their appearance, then amphibians, reptiles, mam- 

 mals, and finally Man. Each of these groups is of a higher order 

 than the last, and the species in each group become more 

 complex as the present era is approached, a fact not shown in 

 the table. 



Even more convincing are the facts revealed by a study of the 

 ancestors of a single species, such as the common horse, Equus 

 (Fig. 159). The horses of the Eocene period are represented by 

 Orohippus, an animal possessing four-toed fore feet and three-toed 

 hind feet. This form was replaced by Mesohippus in the Lower 

 Miocene; then Protohippus appears in the Lower Pliocene, Plio- 

 hippus in the Pliocene, and finally the Equus of recent times. In 

 these forms there has been a gradual degeneration of certain 



