THE EVIDENCE FOR EVOLUTION 725 



(Fig. 35.3). In the latter part of the Cretaceous the interior of the 

 North American continent was further submerged and cut into two by 

 the union of a bay from the Gulf of Mexico and one from the Arctic 

 Sea. The Rocky Mountain revolution ended the Cretaceous with the 

 upheaval of the Rockies, Alps, Himalayas and Andes mountains. The 

 Mesozoic is characterized by the tremendous evolution, diversification 

 and specialization of the reptiles, and is commonly called the Age of 

 Reptiles. Mammals originated in the Triassic and birds in the Jurassic. 

 Most of the modern orders of insects appeared in the Triassic, and snails, 

 bivalve molluscs and sea urchins underwent important evolutionary ad- 

 vances. 



At the end of the Cretaceous a great many reptiles became extinct; 

 they were apparently unable to adapt to the marked changes brought 

 about by the Rocky Mountain revolution. As the climate became colder 

 and drier many of the plants which served as food for the herbivorous 

 reptiles disappeared. Some of the herbivorous reptiles were too large 

 to walk about on land when the swamps dried up. The smaller, warm- 

 blooded mammals Avhich appeared were better able to compete for food, 

 and many of these ate reptilian eggs. The demise of the many kinds of 

 reptiles was probably the result of a combination of a whole host of 

 factors, rather than any single one. 



The Cenozo/c Era. The Cenozoic era, extending from the Rocky 

 Mountain revolution to the present, is subdivided into the earlier Ter- 

 tiary period, which lasted some 74,0()0,()()0 years, and the present Quarter- 

 nary period, which includes the last million or million and one-half 

 years. 



The Tertiary is subdivided into five epochs, the Paleocene, Eocene, 

 Oligocene, Miocene and Pliocene. The Rockies, formed at the begin- 

 ning of the Tertiary, were considerably eroded by the Oligocene, and 

 the North American continent had a gently rolling topography. Another 

 series of uplifts in the Miocene raised the Sierra Nevadas and a new set 

 of Rockies, and resulted in the formation of the western deserts. The 

 climate of the Oligocene was rather mild, and palm trees grew as far 

 north as Wyoming. The uplifts of the Miocene and Pliocene, and the 

 successive ice ages of the Pleistocene, killed off many of the mammals 

 that had evolved. 



The last elevation of the Colorado Plateau, which initiated the 

 cutting of the Grand Canyon, occurred almost entirely in the short 

 Pleistocene and Recent epochs, the two subdivisions of the Quaternary 

 period. Four periods of glaciation occurred in the Pleistocene, between 

 which the sheets of ice retreated. At their greatest extent, these ice 

 sheets extended as far south as the Missouri and Ohio rivers and cov- 

 ered 4,000,000 square miles of North America. The Great Lakes, which 

 were carved out by the advancing glaciers, changed their outlines and 

 connections several times. It is estimated that at one time, when the 

 Mississippi river drained lakes as far west as Duluth and as far east as 

 Buffalo, its volume was more than 60 times as great as at present. 

 During the Pleistocene glaciations enough water was removed from 

 the oceans and locked in the vast sheets of ice to lower the water level 



