so weighty,
that we can hardly doubt that, if the time since the Cambrian epoch is
correctly estimated at 200 millions of years, the date of the commencement
of life on the earth cannot be much less than 500 millions; while it may
not improbably have been longer, because the reaction of {102} the organism
under changes of the environment is believed to have been less active in
low and simple, than in high 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 increase 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.[32] 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 Strata 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
_debris_ 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 universality of this destructive agency. It is
no less clearly shown by the way in which almost every drop 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 {103} of the country, must
always pass from a higher to a lower level, and must