THE LAW OF EVOLUTION CONTINUED. 343 



veins, the production of endless dislocations and irregulari- 

 ties. Again, geologists teach us that the Earth's 

 surface has been growing more varied in elevation — that 

 the most ancient mountain systems are the smallest, and 

 the Andes and Himalayas the most modern; while, in all 

 probability, there have been corresponding changes in the 

 bed of the ocean. As a consequence of this ceaseless mul- 

 tiplication of differences, we now find that no considerable 

 portion of the Earth's exposed surface, is like any other 

 portion, either in contour, in geologic structure, or in 

 chemical composition; and that, in most parts, the surface 

 changes from mile to mile in all these characteristics. 



There has been simultaneously going on a gradual dif- 

 ferentiation of climates. As fast as the Earth cooled and 

 its crust solidified, inequalities of temperature arose be- 

 tween those parts of its surface most exposed to the sun 

 and those less exposed; and thus in time there came to 

 be the marked contrasts, between regions of perpetual ice 

 and snow, regions where winter and summer alternately 

 reign for periods varying according to the latitude, and 

 regions where summer follows summer with scarcely 

 an appreciable variation. Meanwhile, elevations 



and subsidences, recurring here and there over the Earth's 

 crust, tending as they have done to produce irregular dis- 

 tribution of land and sea, have entailed various modifica- 

 tions of climate beyond those dependent on latitude; while 

 a yet further series of such modifications has been produced 

 by increasing differences of height in the lands, which 

 have in sundry places brought arctic, temperate, and tropi- 

 cal climates to within a few miles of one another. The 

 general results of these changes are, that every extensive 

 region has its own meteorologic conditions, and that every 

 locality in each region differs more or less from others in 

 those conditions : as in its structure, its contour, its soil. . 



Thus, between our existing Earth, the phenomena of 

 whose varied crust neither geographers, geologists, min- 



