E\'()LUTI()X IN (iENERAL 465 



and finally becomes cold and firm to the core. This view of 

 world evolution is called the nebular hypothesis. ^ 



AccorcUng to the nebular hypothesis, as the molten in- 

 terior of our earth lost heat it shrunk away from the solid 

 crust, which, following it warped and wrinkled in an uneven 

 way somewhat as the skin of a drying apple wrinkles to fit 

 the shrinking pulp. When the earth was cool enough at the 

 surface to permit condensation of the atmospheric watery 

 vapor and its fall as rain, seas began to form in depressions 

 between the upheaved regions of dry land. Subterranean 

 forces, connected with the further loss of heat, continued to 

 wrinkle the land into chains of mountains. Meanwhile 

 storms, controlled by heat from the sun, brought water to the 

 highlands from the sea to which it returned in streams cutting 

 through the land and carving the surface into varied shapes. 

 The rock waste carried seaward settled off shore, as layers 

 of gravel, sand, or mud. These deposits in time became 

 compacted into solid rock and were slowl.y upheaved again 

 above the level of the sea. This new land was again washed 

 into the sea or may have sunk beneath it and i)een covered 

 by newer washings which later may have been again upraised. 

 From such working over of the crust, most of the land, with 

 its many layers of rock or soil (which is rock waste on its 

 seaward way) came to be as it is. From the many changes 

 thus wrought — some gradual, some sudden — involving wide 

 sway of air and water currents, and the continual though 

 slow redistribution of rock materials — from all this has 

 resulted a greater and greater variety of climate and soil — 

 in a word, a progressive differentiation of the conditions 

 affecting life. This differentiation represents more and more 



' A rival view Icnown as the planetesimal hypothesis has of late 3'ears 

 been gaining ground among geologists. This differs from the nebular 

 hypothesis in supposing that such a solar system as our own evolves 

 by the slow aggregation of innumerable small cold solid bodies (plane- 

 tesimals) moving through space in rings or orbits like those of our planets. 

 They arc consequently drawn together without much violence into 

 larger and larger masses by mutual attraction until there is formed a 

 central sun and planets none of which at any time are altogether gaseous 

 or liquid. Once these larger spheres are formed, other forces than those 

 of mere shrinliing with loss of heat are assumed to account for such 

 geologic changes as those of which we have evidence. 



