THE ALLIES OF THE SQUIRREL 433 



tection to many of our game animals. In separate reserva- 

 tions, as well as in parks and forests, many of our big-game 

 animals are being given a new lease of life and kept from 

 extermination. Bisons or buffaloes, elk, antelopes, and deer 

 have been given special attention, and reservations have 

 been set apart for their protection. When natural condi- 

 tions demand it, the herds are supplied with food and thus 

 kept from starving in severe winters. 



Mammals of Past Ages. With the upheaval of the Rocky 

 Mountain system at the close of Mesozoic time the North 

 American continent assumed practically its present out- 

 line, with the exception of a strip along the southeastern 

 coast, which was still beneath the level of the ocean. In 

 connection with this disturbance of level and the accom- 

 panying climatic changes the great reptiles so characteris- 

 tic 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 ap- 

 peared before the age which bears its name, so the first 

 mammals of which we have any knowledge are to be cred- 

 ited to the Age of Reptiles. They were nearly all of small 

 size and allied to the marsupials and monotremes of today, — 

 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. It is from 

 these deposits that much of our information regarding the 

 early mammals of America has been obtained. The Ameri- 

 can 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. They 

 possess the characteristics of several different groups of to- 

 day, without much modification to a particular kind of life 

 or food. It is among such animals that we must look for the 



