116 WORLD-POWER AND EVOLUTION 



appear at the bottom, reading from the oldest on the left to the 

 latest on the right. The figures in the body of the diagram show 

 the lapse of time in millions of years according to BarrelPs scale. 

 Thus the diagram begins about fifty-five million years ago. The 

 numbers on the right show the percentage of mammalian genera 

 that were extinguished per hundred thousand years. Of course 

 new forms keep coming in to replace the old ones, so that the total 

 number tends on the whole to increase. The rate of extinction is 

 indicated by the solid line. If our knowledge of the past were 

 fuller the line would lie lower in its earlier portions. The striking 

 fact about the line as a whole is the sudden way in which it rises 

 at the right-hand end. This means that during the Glacial Period 

 (marked PI.), the mammals were extinguished at least three times 

 as rapidly as during most of the millions of years that preceded. 

 When the mammals had reached a condition of complete domi- 

 nance they suddenly were wiped out wholesale. In North America 

 the whole family of horses was destroyed; the elephant tribe 

 including the mammoth and mastodon disappeared; the camel 

 which had formerly been abundant passed away leaving no trace 

 save his bones. Still other great families such as the giant beaver, 

 the sloth, the tapirs, and the so-called glyptodonts were like- 

 wise exterminated. In Europe there was a similar appalling 

 destruction of life. 



Directly or indirectly all this destruction arose from the severe 

 climatic oscillations of the Glacial Period, for this one period 

 included four great "epochs." Indeed, if we include all the indirect 

 effects of that period we must include man's part in destroying 

 living creatures. It was apparently the Glacial Period which 

 chiefly stimulated man's mental development and caused his intel- 

 ligence to dominate the earth. In recent times his activities during 

 a period which is but a day geologically are causing the wholesale 

 extinction of all sorts of animals such as bears, wolves, foxes, 

 lynxes, weasels, buffaloes, mountain sheep, and the like. Not all 

 the larger wild animals are gone yet, but they are fast disappear- 



