Such are the beliefs which modern physical geologists hold, or, at any
rate, tend towards holding. But, in so doing, it is obvious that they
by no means prejudge the question, as to what the physical condition of
the globe may have been before our chapters of its history begin, in
what may be called (with that licence which is implied in the often-used
term "prehistoric epoch") "pre-geologic time." The views indicated, in
fact, are not only quite consistent with the hypothesis, that, in the
still earlier period referred to, the condition of our world was very
different; but they may be held by some to necessitate that hypothesis.
The physical philosopher who is accurately acquainted with the velocity
of a cannon-ball, and the precise character of the line which it
traverses for a yard of its course, is necessitated by what he knows of
the laws of nature to conclude that it came from a certain spot, whence
it was impelled by a certain force, and that it has followed a certain
trajectory. In like manner, the student of physical geology, who fully
believes in the uniformity of the general condition of the earth
through geologic time, may feel compelled by what he knows of causation,
and by the general analogy of nature, to suppose that our solar system
was once a nebulous mass; that it gradually condensed, that it broke up
into that wonderful group of harmoniously rolling balls we call planets
and satellites, and that then each of these underwent its appointed
metamorphosis, until at last our own share of the cosmic vapour passed
into that condition in which we first meet with definite records of its
state, and in which it has since, with comparatively little change,
remained.
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