104 COSMIC nilLOSOrHY. [pt. a. 



temperature. And because the amount of solar radiance 

 retained depends 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 definite- 

 ness and permanence 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 deflniteness 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, more- 

 over, evolution has brought about a climatic interdependence. 

 The dependence of terrestrial temperature upon the supply 

 and distribution of solar radiance, has entailed a further 

 dependence of local temperatures upon one another. For 

 example the warm temperature of southern Europe is largely 

 dependent on the hot dry winds which blow from Sahara, 

 and which powerfully assist in melting the glaciers of the 

 Alps. If Sahara were to be submerged — as indeed it has 

 been at a recent epoch — these dry winds would be replaced 

 by cooler winds charged with vapour, which would condense 

 into snow on the Alps, and thus enlarge the glaciers already 

 formed there, instead of melting them away. Thus the 

 climate would be changed throughout Europe, and the 

 direction of winds would be altered over a still larger area 

 of the globe. If Lapland and the isthmus of Panama were 

 to subside at the same time, so that icebergs could float 



