ch. v.] THE EVOLUTION OF TEE EARTH. 405 



through the Baltic to the coast of Prussia, while the Gulf 

 Stream would be diverted into the Pacific Ocean, the climate 

 of Europe might become glacial. Yet either the submergence 

 of Greenland, or the elevation of the East Indian Archi- 

 pelago into a continuous continent, would perhaps suffice to 

 neutralize all these agencies, and restore the genial warmth. 

 In such climatic relations we see vividly illustrated that 

 kind of integration which brings the condition of each part 

 of an aggregate into dependence upon the condition of all 

 the other parts. 



It is now sufficiently proved that the development of the 

 earth, like the development of the planetary system to which 

 it belongs, has been primarily an integration of matter and 

 dissipation of motion, and secondarily a change from in- 

 definite homogeneity with relative isolation of parts to 

 definite heterogeneity with relative interdependence among 

 parts. But our survey of telluric evolution is as yet far 

 from complete. While enough has been said concerning the 

 redistributions of matter which have gone on over the face 

 of the globe, nothing has been said concerning the far move 

 wonderful and interesting redistributions of the molecular 

 motion which the earth is continually receiving from the 

 sun. Here, as already briefly hinted, we have the chief 

 source of terrestrial heterogeneity. In the chapter on the 

 Law of Evolution, it was observed, as a general truth, that 

 homogeneous forces incident upon a heterogeneous aggregate 

 undergo differentiation and integration. We shall now find 

 this general truth beautifully exemplified in the history of 

 the surface of our planet. At a remote era in that history, 

 the differentiation and integration of solar radiance began 

 gradually to constitute the most important part of the com- 

 plex process of terrestrial evolution. We have now to show 

 how this has been done ; and we shall find it desirable to 

 introduce the subject with an inquiry into the Sources of 

 Terrestrial Energy. 



