428 GENERAL ZOOLOGY 



Geological Development of Mammals. With the upheaval 

 of the Rocky Mountain system at the close of Mesozoic 

 time the North American continent assumed practically its 

 present outline, with the exception of a strip along the south- 

 eastern coast, which was still beneath the level of the ocean 

 (see map, p. 363). In connection with this disturbance of 

 level and the accompanying climatic changes the great rep- 

 tiles so characteristic of the period became extinct and left 

 the field clear for the development of the mammals. The 

 succeeding period is therefore called the Age of Mammals. 

 As we have already seen, representatives of each group of 

 animals have appeared before the age which bears its name, 

 so the first mammals of which we have any knowledge are 

 to be credited to the Age of Reptiles. They were nearly all 

 of small size and allied to the marsupials and monotremes 

 of to-day, groups which we have noticed as being the 

 lowest of the class. 



During the Age of Mammals there were extensive areas 

 of fresh- water lakes in western North America, shown by the 

 shaded areas on the map on page 363. It is from these deposits 

 that much of our information regarding the early mammals 

 of America has been obtained. The American Museum of 

 Natural History in New York City has on exhibition a large 

 series of fossils from this region. Many of the specimens 

 discovered are of generalized structure; that is, they possess 

 the characteristics of several different groups of to-day, with- 

 out much special adaptation or modification to a particular 

 kind of life or food. It is among such animals that we must 

 look for the ancestors of the species of to-day, for no species 

 of mammal which was in existence in the Age of Mammals 

 has lasted through to the present time. 



Of no other animal have we so complete and satisfactory 

 a geological record as in the case of the horse. From fossil 

 remains found in the western part of the United States we 



