EVOLUTION IN GENERAL 465 
and finally becomes cold and firm to the core. This view-of 
world evolution is called the nebular hypothesis.' 
According 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 slowly upheaved again 
above the level of the sea. This new land was again washed 
into the sea or may have sunk beneath it and been 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 
1 A rival view known as the planetesimal hypothesis has of late years 
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 are 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 shrinking with loss of heat are assumed to account for such 
geologic changes as those of which we have evidence. 
