80 



ceeded by the frigid conditions of the Permian. What 

 number of such cycles have filled the space between, and 

 what have been the causes, we have only the vaguest 

 notion. Another factor of plant-distribution, the oscilla- 

 tion between land and sea, giving freedom of migration 

 at one time with close isolation at another, we have but 

 scanty information of. A third change that must have 

 largely influenced plant life, condition of atmosphere, is 

 almost a closed book. There is one thing certain -the 

 present constitution of the atmosphere, however agreeable 

 to us, is far from the best for plant life. There are at 

 present on the average but three and a half parts of car- 

 bonic acid in ten thousand parts of air, yet plants do best 

 when this gas is present to an extent even exceeding four 

 parts in a hundred, or more than a hundred times as much 

 of this gas. The presence of a larger proportion of this 

 gas than at present obtains would not only afford more 

 food for the plants, but would much modify climatic con- 

 ditions, rendering it warmer and more equitable. It is 

 probable that atmospheric conditions, distribution of land 

 and sea, together with large cycles due to other causes, 

 have caused oscillations of climate of which we can form 

 but the slightest conception. Thus the evolution and dis- 

 tribution of plants is a far more profound problem than it 

 is generally considered. 





