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146 EVOLUTION OF THE EARTH 



the derived human line Is broken to the point ii, the first 

 actual record of man; the descent from the trees Is, however, 

 made coincident with the Himalayan uplift and with the In- 

 creasing aridity of the Miocene. 



Man's Intellectual and spiritual rise and his dominance over 

 the forces of nature and over the brute creation are shown 

 graphically by the height to which his line ascends above the 

 highest crest of the evolutionary crises which have gone before. 

 For this dominance the Cascadlan revolution with its recurrent 

 periods of devastating cold, out of which we are emerging, 

 must be looked upon as a contributory cause. 



Thus time has wrought great changes in earth and sea, and 

 these changes, acting directly or through climate, have always 

 found somewhere in the unending chain of living beings certain 

 groups whose plasticity permitted their adaptation to the newly 

 arising conditions. 



The great heart of nature beats, its throbbing stimulates the 

 pulse of life, and not until that heart is stilled forever will the 

 rhythmic tide of evolution cease to flow. 



