from physical considerations,
more especially by Sir William Thomson, which limit the possible
existence of the earth's solid crust to one hundred millions of years.
Similar conclusions have also been deduced from what is known of the
physical constitution of the sun. Croll's own ingenious theory of
glacial periods produced by the varying eccentricity of the earth's
orbit, along with the precession of the equinoxes, would give,
according to him, about 80,000 years ago for the date of the Glacial
period, and for the beginning of the Tertiary period about 3,000,000
years ago.

It would thus appear that physical and geological science conspire in
assigning a great antiquity to the earth, but not an unlimited
antiquity. They agree in restricting the ages that have elapsed since
the introduction of life within one hundred millions of years. I
confess, however, that a consideration of the fact that all our
geological measures of erosion and deposition seem to be based on
cases which refer to what may be termed minimum action leads me to
believe that the actual time will fall very far within this limit. For
example, if we were to suppose an elevation of the land drained by the
Mississippi even to a small amount, its cutting power would be vastly
increased for a long time. The same effect would result from a
subsidence and re-elevation, or from any cause increasing the amount
of rainfall or deposition of snows in winter. Now we know that such
things have occurred in the past, while we have no reason to believe
that the amount of action was ever much less than at present. Similar
considerations apply to nearly all our geological measures of time;
and there has been a tendency to exaggerate these, as if geologists
were entitled to demand unlimited time, and to stretch the doctrine of
uniformity to the utmost.

6. During the whole time referred to by geology, the great laws both
of inorganic and organic nature have been the same as at present. The
evidence of light and darkness, of sunshine and shower, of summer and
winter, and of all the known igneous and aqueous causes of change,
extends back almost, and in some of these cases altogether, to the
beginning of the Palaeozoic period. In like manner the animals and
plants of the oldest rocks are constructed on the same physiological
and anatomical principles with existing tribes, and they can be
arranged in the same genera, orders, or classes, though specifically
distinct. The re