Our genealogical tree fairly bristles with
branches, but the branches do not seem to form any regular whorls or
spirals. Few of them have developed into more than feeble growths.
They now contain generally but few species. Many of them are
largely or entirely parasitic, and in connection with this mode of
life have undergone modifications and degeneration which make it
exceedingly difficult to decipher their descent or relationships.
Four of these branches have reached great prominence in numbers and
importance. One or two others were formerly equally numerous and
have since become almost extinct; so the brachiopoda, which have
been almost entirely replaced by mollusks. The same may very
possibly be true of others. For of the amount of extinction of
larger groups we have generally but an exceedingly faint conception.
Indeed in this respect the worms have been well compared to the
relics which fill the shelves of one of our grandmother's
china-closets.
The four great branches are the echinoderms, mollusks, articulates,
and vertebrates. The echinoderms, including starfishes, sea-urchins,
and others straggled early from the great army. We know as yet
almost nothing of their history; when deciphered it will be as
strange as any romance. The vertebrates are of course the most
important line, as including the ancestors of man.
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