COSMIC PHILOSOPHY 



solidification of its crust, our planet must have 

 been comparatively homogeneous in tempera- 

 ture, owing to the circulation which is always 

 maintained in masses of heated fluid. The sur- 

 face portions must, however, have been some- 

 what cooler than the interior, and this differ- 

 ence would be rendered more definite by the 

 formation of the crust, and by the subsequent 

 separation of the ocean from the gaseous atmos- 

 phere. As the contour of land and sea became 

 more definite and more permanent, differences in 

 temperature between different parts of the sur- 

 face must likewise have become more decided. 

 Nevertheless the chief cause of climatic differen- 

 tiations — the inclination of the earth's axis — 

 did not begin to produce its most conspicuous 

 effects until a later period. As long as our planet 

 retained a great proportion of its primitive heat, 

 there could have been little difference between 

 winter and summer, or between the temperature 

 at the poles and at the equator. But when the 

 earth had lost so much heat that its external 

 temperature began to depend chiefly upon the 

 supply of solar radiance, then there commenced 

 a gradual differentiation of climates. There be- 

 gan to be a marked difference between summer 

 and winter, and between arctic, temperate, and 

 tropical zones. And now also the distribution 

 of land and sea began to produce climatic ef- 

 fects, owing to the fact that solar radiance is 

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