history, first of the changes in its physical structure, next of the
mutations in the organic forms that have, in serial order, appeared in
the successive strata of its external envelope, from the period of that
far distant crisis when it was a molten globe on which its primitive
granitic covering was just beginning to concrete, in consequence of
abating heat, until we have arrived at the first prognostic signs of
approaching human existence.

The rock upon rock of vast thickness, by which the earth's crust,
through countless ages, has been formed, unquestionably constitutes a
most extraordinary phenomenon of physical creation, but hardly so
marvellous and incomprehensible as the beginning, progress, and end of
the divers orders of marine and terrestrial beings that filled each
world of life. It is to geologists, to PLAYFAIR, HUTTON, LYELL,
BUCKLAND, SEDGWICK, OWEN, and other great names, native and foreign, to
whom we are indebted for this singular revelation of Nature's works. It
is their unwearied research that has opened to us the surprising
spectacle we have attempted briefly to describe of the diversified
groups of species which have, in the course of the earth's history,
succeeded each other at vast intervals of time; one set of animals and
plants wholly or partly disappearing from the face of our planet, and
others, which apparently did not before exist, becoming the only or
predominant occupants of the globe.

Now the great question arises--whence, by what power, or by what law,
were these reiterated transitions brought about? Were the organized
species of one geological epoch, by some long-continued agency of
natural causes, transmuted into other and succeeding species? or were
there an extinction of species, and a replacement of them by others,
through special and miraculous acts of creation? or, lastly, did species