304 GENERAL ZOOLOGY 



brates in any numbers. The lack of fossils in these early rocks 

 may be due in part to the fact that the conditions for fossilization 

 preclude the possibility of the preservation of many forms, partic- 

 ularly soft-bodied forms. This, of course, holds true for all 

 succeeding strata and accounts for gaps in our knowledge of the 

 ancient history of soft-bodied progenitors of modern groups of 

 animals. Beginning with the Paleozoic rocks the material 

 becomes richer in fossil content and reveals the gradual emer- 

 gence of higher forms as time goes on. The predominant forms 

 of the Paleozoic era are higher invertebrates, fishes, amphibians, 

 and toward the end of the era, primitive reptiles. The Mesozoic 

 era is spoken of as the age of reptiles, because during this period 

 they achieved a greater size and abundance than during any 

 time before or since. During this era also birds and mammals 

 made their first appearance. The Cenozoic era is characterized 

 by a fuller development of mammals and the Psychozoic by the 

 later stages in the evolution of man. 



On an evolutionary basis, the relationships of the various 

 animal groups can be represented graphically in the form of a tree, 

 whose terminal twigs alone represent the living fauna. The rest 

 of the tree structure represents the paths leading to the evolution 

 of the living forms. All of the tree except the twigs therefore 

 represents generations of ancestral forms that have become 

 extinct as species. The proof that such ancestral forms actually 

 existed in the past is furnished by paleontology. But since many 

 ancestral forms have left no record, and since the record of those 

 preserved is often fragmentary, the task of reconstructing 

 ancestral series of any single species with any degree of comple- 

 tion is a very difficult one. Nevertheless, this is being accom- 

 plished in an ever-increasing number of instances, of which one 

 of the best known is the case of the ancient history of the horse. 



Evolution of the Horse. — The earliest known ancestor of the 

 horse lived in the Eocene (epoch) of the Cenozoic era of North 

 America and is known as Hyracotherium or Eohippus. It had 

 four toes and a rudiment of digit I on its forefeet, and three toes 

 and a splint of digit V on its hindfeet. In later strata of the 

 Eocene were found the remains of Protorohippus, somewhat 

 larger than Hyracotherium, with four toes on the forefeet and 

 three on the hindfeet. Mesohippus, the size of a sheep and 

 found in the Oligocene epoch, had three toes and the splint of 



