chalk, which constitutes a great part of the Cretaceous formation in
some parts of the world, is practically identical in its physical and
chemical characters with a substance which is now being formed at the
bottom of the Atlantic Ocean, and covers an enormous area; other beds
of rock are comparable with the sands which are being formed upon
sea-shores, packed together, and so on. Thus, omitting rocks of igneous
origin, it is demonstrable that all these beds of stone, 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 vegetable nature are concerned,
it has been preceded by a different condition. We can pursue this
evidence until we reach the