MATTHEW. CLIMATE AND EVOLUTION 



177 



these periods will be especially adapted. The periods of continental 

 emergence were periods of arid and markedly zonal climate, and the 



fauna9 must adapt themselves to these 

 while favoring the spread and wide dis- 

 tribntio]! of races, would l)e unfavora- 

 ble to abundance of life and the ease 

 with which animals could obtain a liv- 

 ing. The animals subjected to them 

 must mamtain themselves against the 

 inclemency of nature, the scarcity of 

 food, the variations of temperature, as 

 well as against the competition of rivals 

 and the attacks of enemies. In the 

 moist tropical climatic phase, animals 

 would find food abundant and tempera- 

 ture relatively constant; but the larger 

 percentage of carbonic acid and prob- 

 ably smaller percentage of oxygen in 

 the atmosphere during those phases 

 would tend to sluggishness. 



We should expect, therefore, to find 

 in the land life adapted to the arid cli- 

 matic phase a greater activity and 

 higher development of life, special 

 adaptations to resist violent changes in 

 temperature and specializations fitting 

 them to the open grassy plains and des- 

 ert life. In the moist tropical phase of 

 land life, we should expect to find 

 adaptations to abundant food, to rela- 

 tively sluggish life and to the great ex- 

 panse of swamp and forest vegetation 

 that should characterize such a phase 

 of climate. 



The oncoming cold and arid condi- 

 tions should appear first at the poles 

 and spread towards the temperate and 

 tropical regions. Owing to the distri- 

 bution of the great land masses, this 

 would involve a general tendency for 

 the great migrations resulting from the 



conditions. Such conditions. 



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