XXIV 



THE EXTINCTION OF CERTAIN SPECIES OF MAMMALS 

 AND THE APPROACHING EXTINCTION OF OTHERS 



Throughout the long geological ages, species, genera, families and 

 orders of plants and animals have developed, flourished and passed 

 away, leaving their recognizable remains in the rocks to tell the story 

 of their former existence. Whole groups of organisms once occupying 

 a prominent position in nature are long since extinct, such, for ex- 

 ample, as the trilobites, calamites, lepidodendrons and sigillarias of 

 the Paleozoic Era ; the ammonites, dinosaurs, ichthyosaurs, plesiosaurs 

 and pterodactyles of the Mesozoic Era; the gigantic mammals, Dino- 

 cerata, of the early Cenoz.oic Era, and many others. Some groups, once 

 highly important, still exist, but in insignificant numbers as compared 

 with the past, as, for example, the brachiopods. 



The Tertiary Age is often called the Age of Mammals. Though 

 primitive forms of mammals appear to have existed very far back in 

 the Mesozoic Era, as shown by small fossil jaws, they did not become 

 at all important until the beginning of Tertiary time. Then they sud- 

 denly expanded, spread over most of the earth's surface, and developed 

 a vast assemblage of distinct types, in this respect reaching their zenith 

 and beginning their decline. Before the close of the Tertiary all the 

 higher mammalian types appeared. Many of the groups that flourished 

 during early and middle Tertiary time became extinct before the end 

 of that period. Others dwindled rapidly during the Pleistocene or Gla- 

 cial Age. Hence the present or "Psychozoic" Epoch presents to us in 

 living form only a remnant of the rich mammalian life of the Tertiary. 



Ancestral camels, horses and rhinoceroses roamed the western plains 

 of the United States during Tertiary time, but dwindled away and 

 disappeared before the dawn of the Recent Epoch, their development 

 to modern types having been continued on other continents. During 

 Pleistocene time mammoths, distinct from but closely related to living 

 species of elephants, ranged almost throughout North America and 

 Europe. Their teeth, tusks and bones are especially plentiful in Alaska 

 and Yukon Territory, and even more abundant in the frozen wastes 

 of Siberia, where their flesh has been found intact, frozen in the perma- 



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