of which a total of not less than seventy thousand feet is
known, have been formed by natural agencies, either out of the
waste and washing of the dry land, or else by the accumulation
of the exuviae of plants and animals. Many of these strata are
full of such exuviae--the so-called "fossils." Remains of
thousands of species of animals and plants, as perfectly
recognisable as those of existing forms of life which you meet
with in museums, or as the shells which you pick up upon the
sea-beach, have been imbedded in the ancient sands, or muds, or
limestones, just as they are being imbedded now, in sandy, or
clayey, or calcareous subaqueous deposits. They furnish us with
a record, the general nature of which cannot be misinterpreted,
of the kinds of things that have lived upon the surface of the
earth during the time that is registered by this great thickness
of stratified rocks. But even a superficial study of these
fossils shows us that the animals and plants which live at the
present time have had only a temporary duration; for the remains
of such modern forms of life are met with, for the most part,
only in the uppermost or latest tertiaries, and their number
rapidly diminishes in the lower deposits of that epoch. In the
older tertiaries, the places of existing animals and plants are
taken by other forms, as numerous and diversified as those which
live now in the same localities, but more or less different from
them; in the mesozoic rocks, these are replaced by others yet
more divergent from modern types; and, in the paleozoic
formations, the contrast is still more marked. Thus the
circumstantial evidence absolutely negatives the conception of
the eternity of the present condition of things. We can say,
with certainty, that the present condition of things has existed
for a comparatively short period; and that, so far as animal and