ects rather take its time from zoology
than the reverse; for if Mr. Darwin's theory be true, past time down to the
deposition of the Upper Silurian strata can have been but a very small
fraction of that during which strata have been deposited. For when those
Upper Silurian strata were formed, organic evolution had already run a
great part of its course, perhaps the longest, slowest, and most difficult
part of that course.

At that ancient epoch not only were the vertebrate, molluscous, and
arthropod types distinctly and clearly differentiated, but highly developed
forms had been produced in each of these sub-kingdoms. Thus in the
Vertebrata there were fishes not belonging to the lowest but to the very
highest groups which are known to have ever been developed, namely, the
Elasmobranchs (the highly organized sharks and rays) and the Ganoids, a
group now poorly represented, but for which the sturgeon may stand as a
type, and which in many important respects more nearly resemble higher
Vertebrata than do the ordinary or osseous fishes. Fishes in which the
ventral fins are placed in front of the pectoral ones (_i.e._ jugular
fishes) have been generally considered to be comparatively modern forms.
But Professor Huxley has kindly informed me that he has discovered a {141}
jugular fish in the Permian deposits.

Amongst the molluscous animals we have members of the very highest known
class, namely, the Cephalopods, or cuttle-fish class; and amongst
articulated animals we find Trilobites and Eurypterida, which do not belong
to any incipient worm-like group, but are distinctly differentiated
Crustacea of no low form.

[Illustration: CUTTLE-FISH.
A. Ventral aspect. B. Dorsal aspect.]

We have in all these animal types nervous systems differentiated on
distinctly different patterns, fully formed organs of circulation,
digestion, excretion, and generation, complexly constructed eyes and other
sense organs; in fact, all the most elaborate and complete animal
structures built up, and not only once, for in the fishes and mollusca we
have (as described in the third chapter of this work) the coincidence of
the independently developed organs of sense attaining a nearly similar
complexity in two quite distinct forms. If, then, so small an advance {142}
has been made in fishes, molluscs, and arthropods since the Upper Silurian
deposits, it will probably be within the mark to consider that the period
before those deposits (during which