THE EVOLUTION OF THE EARTH 



both absorbed and given out more rapidly by- 

 land than by water. Areas of the earth's surface 

 where sea predominated began now to be dis- 

 tinguished from areas where land predominated, 

 by their more equable temperature. And be- 

 cause the amount of solar radiance retained de- 

 pends upon the density of the atmosphere, there 

 ensued differences of climate between mountains 

 and valleys, between table-lands and low-lying 

 plains. Here too the increased heterogeneity 

 was attended by increased definiteness and per- 

 manence of climatic relations. For the thermal 

 variations, depending on the earth's rhythmic 

 change of position with reference to the sun, set 

 up atmospheric currents in definite directions 

 and of tolerably regular recurrence. Sundry of 

 these currents, swayed by the earth's rotatory 

 momentum, became specialized as trade-winds 

 and monsoons ; while in the ocean there went 

 on a similar specialization, as exemplified in the 

 constant course of the Gulf Stream and other 

 marine currents. The definiteness of the total 

 result, as well as its heterogeneity, may be well 

 illustrated by any map of isothermal lines, — 

 bearing in mind, as we must, that during long 

 periods these lines shift only within narrow 

 limits. 



Among the various portions of our earth's 

 surface, moreover, evolution has brought about 

 a climatic interdependence. The dependence of 

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