EXTERNAL FACTORS. 



415 



Earth, to sets of incident forces which differ from previous 

 sets, both by changes in the proportions of the factors, and, 

 occasionally, by the addition of new factors. 



§ 150. Variations in the astronomical conditions joined 

 with variations in the geological conditions, bring about 

 variations in the meteorological conditions. Those extremely 

 slow alternations of elevation and subsidence, which there is 

 reason to believe take place over immense areas, here pro- 

 ducing a continent where once there was a fathomless ocean, 

 and there causing wide seas to spread where in a long past 

 epoch there stood snow-capped mountains, gradually work 

 great atmospheric changes. "While yet the highest parts of 

 an emerging surface of the Earth's crust, exist as a cluster of 

 islands, the plants and animals which in course of time migrate 

 to them, have climates that are peculiar to small tracts of 

 land surrounded by large tracts of water. As, by successive 

 upheavals, greater areas are exposed, there begin to arise 

 sensible contrasts between the states of their peripheral parts 

 and their central parts : the sea and land breezes, which 

 daily moderate the extremes of temperature near the shores, 

 cease to affect the interiors ; and the interiors, less qualified 

 too in their heat and cold by such ocean-currents as bathe 

 the shores, acquire more decidedly the characters due to 

 their latitudes. Along with the further elevations which 

 unite the members of the archipelago into a continent, there 

 come new meteorologic changes, as well as exacerbations of 

 the old. The winds, which were comparatively uniform in 

 their directions and periods when only islands existed, grow 

 involved in their distribution, and widely- different in dif- 

 ferent parts of the continent. The quantities of rain which 

 they discharge and of moisture which they absorb, vary 

 everywhere according to the proximity to the sea and to 

 surfaces of land having special characters. 



Other complications result from variations of height above 

 the sea : elevation producing a decrease of heat and conse- 



