owever, of these ancient rocks
at once reduce the theory of progressive development to within very
narrow limits, for already they comprise a very full representation of
the radiata, mollusca, and articulata proper to the sea. Thus, in the
great division of radiata, we find asteriod and helianthoid zoophytes,
besides crinoid and cystidean echinoderms. In the mollusca, between 200
and 300 species of cephalopoda are enumerated. In the articulata we have
the crustaceans represented by more than 200 species of trilobites,
besides other genera of the same class. The remains of fish are as yet
confined to the upper part of the Silurian series; but some of these
belong to placoid fish, which occupy a high grade in the scale of
organization. Some naturalists have assumed that the earliest fauna was
exclusively marine, because we have not yet found a single Silurian
helix, insect, bird, terrestrial reptile or mammifer; but when we carry
back our investigation to a period so remote from the present, we ought
not to be surprised if the only accessible strata should be limited to
deposits formed far from land, because the ocean probably occupied then,
as now, the greater part of the earth's surface. After so many entire
geographical revolutions, the chances are nearly three to one in favor
of our finding that such small portions of the existing continents and
islands as expose Silurian strata to view, should coincide in position
with the ancient ocean rather than the land. We must not, therefore,
too hastily infer, from the absence of fossil bones of mammalia in the
older rocks, that the highest class of vertebrated animals did not exist
in remoter ages. There are regions at present, in the Indian and Pacific
Oceans, coextensive in area with the continents of Europe and North
America, where we might dredge the bottom and draw up thousands of
shells and corals, without obtaining one bone of a land quadruped.
Suppose our mariners were to report, that, on sounding in the Indian
Ocean near some coral reefs, and at some distance from the land, they
drew up on hooks attached to their line portions of a leopard, elephant,
or tapir, should we not be skeptical as to the accuracy of their
statements? and if we had no doubt of their veracity, might we not
suspect them to be unskilful naturalists? or, if the fact were
unquestioned, should we not be disposed to believe that some vessel had
been wrecked on the spot?

The casualties must always b