general aspect and character, they were
all, if I may so speak, equally underlaid by the great leading ideas
which still constitute the master types of animal life. And these
leading ideas are four in number. _First_, there is the _star-like_ type
of life,--life embodied in a form that, as in the corals, the
sea-anemones, the sea-urchins, and the star-fishes, radiates outwards
from a centre; _second_, there is the _articulated_ type of life,--life
embodied in a form composed, as in the worms, crustaceans, and insects,
of a series of rings united by their edges, but more or less moveable on
each other; _third_, there is the bilateral or _molluscan_ type of
life,--life embodied in a form in which there is a duality of
corresponding parts, ranged, as in the cuttle-fishes, the clams, and
the snails, on the sides of a central axis or plane; and _fourth_, there
is the _vertebrate_ type of life,--life embodied in a form in which an
internal skeleton is built up into two cavities placed the one over the
other; the upper for the reception of the nervous centres, cerebral and
spinal,--the lower for the lodgment of the respiratory, circulatory, and
digestive organs. Such have been the four central ideas of the faunas of
every succeeding creation, except perhaps the earliest of all, that of
the Lower Silurian System, in which, so far as is yet known, only three
of the number existed,--the radiated, articulated, and molluscan ideas
or types. That Omnipotent Creator, infinite in his resources,--who, in
at least the details of his workings, seems never yet to have repeated
himself, but, as Lyell well expresses it, breaks, when the parents of a
species have been moulded, the dye in which they were cast,--manifests
himself, in these four great ideas, as the unchanging and unchangeable
One. They serve to bind together the present with all the past; and
determine the unity of the authorship of a wonderfully complicated
design, executed on a groundwork broad as time, and whose scope and
bearing are deep as eternity.

The fauna of the Silurian System bears in all its three great types the
stamp of a fashion peculiarly antique, and which, save in a few of the
mollusca, has long since become obsolete. Its radiate animals are
chiefly corals, simple or compound, whose inhabitants may have somewhat
resembled the sea-anemones; with zoophites, akin mayhap to the sea-pens,
though the relationship must have been a remote one; and numerous
crinoids,