which, however, sufficiently
indicate, from their resemblance to the corresponding parts of an
existing shark,--the cestracion,--that they belonged to fishes furnished
with the two pairs of fins now so generally recognized as the homologues
of the fore and hinder limbs in quadrupeds. With the second earliest
vertebrates,--the ganoids of the Old Red Sandstone,--we are more
directly acquainted, and know that they exhibited the true typical
form,--a vertebral column terminating in a brain-protecting skull; and
that, in at least the acanth, celacanth, and dipterian families, they
had the limb-like fins. In the upper parts of the system the earliest
reptiles leave the first known traces of the typical foot, with its five
digits. Higher still in one of the deposits of the Trias we are startled
by what seems to be the impression of a human hand of an uncouth massive
shape, but with the thumb apparently set in opposition, as in man, to
the other fingers; we next trace the type upwards among the wonderfully
developed reptiles of the Secondary periods; then among the mammals of
the Tertiary ages, higher and yet higher forms appear; the mute
prophecies of the coming being become with each approach clearer,
fuller, more expressive, and at length receive their fulfilment in the
advent of man. A double meaning attaches to the term type; and hence
some ambiguity in the writings which have appeared on this curious
subject. Type means a prophecy embodied in symbol; it means also what
Sir Joshua Reynolds well terms "one of the general forms of nature,"--a
pattern form, from which all others in the same class or family, however
numerous, are recognized as mere exceptions and aberrations. But in the
geologic series both meanings converge and become one. The form or
number typical as the _general_ form or number, is found typical also as
a _prophecy_ of the form or number that came at length to be exemplified
in the deputed lord of creation. Let us in our examples take typical
numbers, as more easily illustrated without diagrams than typical forms.

There are vertebrate animals of the second age of ichthyic existence,
that, like the _Pterichthys_ and _Coccosteus_, were furnished with but
two limbs. The muraenidae of recent times have no more; at least one of
their number, the muraena proper, wants limbs altogether; so also do the
lampreys. The snakes are equally limbless, save that the boas and
pythons possess the rudiments of a single pair;