EXTERNAL FACTORS. 503 



in the course of generations, subjected by alterations in the 

 crust of the Earth, to sets of incident forces differing from 

 previous sets, both by changes in the proportions of the 

 factors and, occasionally, by the addition of new factora. 



§ 150. Variations in the astronomical conditions joined 

 with variations in the geological conditions, bring about 

 variations in the meteorological conditions. Those slow 

 alternations of elevation and subsidence which take place 

 over immense areas, here producing 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 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, b}^ successive upheavals, 

 greater areas are exposed, there begin to arise sensible con- 

 trasts between the states of their peripheral parts and their 

 central parts. The 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 approach the coast, 

 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 meteoro- 

 logic 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 different 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 

 33 



