THE EARTH'S AGE 213 



the organism under changes of the environment is believed 

 to have been less active in low and simple, than in hio-h 

 and complex forms of life, and thus the processes of organic 

 development may for countless ages have been excessively 

 slow. 



But according to the physicists, no such periods as are 

 here contemplated can be granted. From a consideration 

 of the possible sources of the heat of the sun, as well as 

 from calculations of the period during which the earth can 

 have been cooling to bring about the present rate of in- 

 crease of temperature as we descend beneath the surface. 

 Sir William Thomson concludes that the crust of the 

 earth cannot have been solidified much longer than 100 

 million years (the maximum possible being 400 millions), 

 and this conclusion is held by Dr. Croll and other men of 

 eminence to be almost indisputable.^ It will therefore be 

 well to consider on what data the calculations of geologists 

 have been founded, and how far the views here set forth, 

 as to frequent changes of climate throughout all geological 

 time, may affect the rate of biological change. 



Denudation and Deposition of Stratct as a Measure of 

 Time. — The materials of all the stratified rocks of the 

 globe have been obtained from the dry land. Every point 

 of the surface is exposed to the destructive influences of 

 sun and wind, frost, snow, and rain, which break up and 

 wear away the hardest rocks as well as the softer deposits, 

 and by means of rivers convey the worn material to the 

 sea. The existence of a considerable depth of soil over the 

 greater part of the earth's surface ; of vast heaps of rocky 

 ddhris at the foot of every inland cliff; of enormous deposits 

 of gravel, sand, and loam ; as well as the shingle, pebbles, 

 sand or mud, of every sea-shore, alike attest the univer- 

 sality of this destructive agency. It is no less clearly shown 

 by the way in which almost every dro]) of running water — 

 whether in gutter, brooklet, stream or large river — becomes 

 discoloured after each heavy rainfall, since the matter which 

 causes this discolouration must be derived from the surface 



1 Trans. Royal Society of Edinhurgh, Vol. XXIIL p. 161. Quarterly 

 Journal of Science, 1877. (Croll on the "Probable Origin and Age of the 



Sun.") 



