to draw
its picture. In other cases such traces alone remain as the impression
which the feet of animals have left on wet sand or mud over which they
passed when alive, or the remains of their undigested food (coprolites).

The analytical study of the animal and vegetable kingdoms of the
primitive world has given rise to two distinct branches of science; one
purely morphological, which occupies itself in natural and physiological
descriptions, and in the endeavour to fill up from extinct forms the
chasms which present themselves in the series of existing species; the
other branch, more especially geological considers the relations of the
fossil remains to the superposition and relative age of the sedimentary
beds in which they are found. The first long predominated; and the
superficial manner which then prevailed of comparing fossil and existing
species led to errors of which traces still remain in the strange
denominations which were given to certain natural objects. Writers
attempted to identify all extinct forms with living species, as, in the
sixteenth century, the animals of the New World were confounded by false
analogies with those of the Old.

In studying the relative age of fossils by the order of superposition of
the strata in which they are found, important relations have been
discovered between families and species (the latter always few in
numbers) which have disappeared and those which are still living. All
observations concur in showing that the fossil floras and faunas differ
from the present animal and vegetable forms the more widely in
proportion as the sedimentary beds to which they belong are lower, or
more ancient.

Thus great variations have successively taken place in the general
types of organic life, and these grand phenomena, which were first
pointed out by Cuvier, offer numerical relations which Deshayes and
Lyell have made the object of researches by which they have been
conducted to important results, especially as regards the numerous and
well-preserved fossils of the Tertiary formation. Agassiz, who has
examined 1,700 species of fossil fishes, and who estimates at 8,000 the
number of living species which have been described, or which are
preserved in our collections, affirms that, with the exception of one
small fossil fish peculiar to the argillaceous geodes of Greenland, he
has never met in the Transition, Secondary, or Tertiary strata with any
example of this class specifically iden