26 THE ORIGIN AND EVOLUTION OF LIFE 



result of the combined action of the hthosphere (the rocks), 

 hydrosphere (the water), and atmosphere (the air). Liquefac- 

 tion of the rocks occurred locally and occasionally as the result 

 of heat generated by increased pressure and by radioactivity; 

 but the planetesimal hypothesis assumes that the present 

 elastic rigid condition of the earth prevailed — at least in its 

 outer half — throughout the history of its growth from the small 

 original nebular knot to its present proportions and caused the 

 permanence of its continents and of its oceanic basins. We 

 are thus brought to conditions that are fundamental to the 

 evolution of life on the earth. According to the opinion of 

 Chamberlin, cited by Pirsson and Schuchert,^ life on the earth 

 may have been possible when it attained the present size of 

 Mars. 



According to Becker,- who follows the traditional theory of 

 a primitive molten globe, the earth first presented a nearly 

 smooth, equipotential surface, determined not by its mineral 

 composition, but by its density. As the surface cooled down 

 a temperature was reached at which the waters of the gaseous 

 envelope united with the superficial rocks and led to an aqueo- 

 igneous state. After further cooling the second and final con- 

 sohdation followed, dating the origin of the granites and grani- 

 tary rocks. The areas which cooled most rapidly and best 

 conducted heat formed shallow oceanic basins, whereas the 

 areas of poor conductivity which cooled more slowly stood out 

 as low continents. The internal heat of the cooling globe still 

 continues to do its work, and the cyclic history of its surface 

 is completed by the erosion of rocks, by the accumulation of 

 sediments, and by the following subsidence of the areas loaded 



' Pirsson, Louis V., and Schuchert, Charles, 1915, p. 535- 

 - George F. Becker, letter of October 15, 1915. 



