n species. For instance, I cannot doubt that all the
Silurian trilobites have descended from some one crustacean, which must
have lived long before the Silurian age, and which probably differed
greatly from any known animal. Some of the most ancient Silurian animals,
as the Nautilus, Lingula, &c., do not differ much from living species; and
it cannot on my theory be supposed, that these old species were the
progenitors of all the species of the orders to which they belong, for they
do not present characters in any degree intermediate between them. If,
moreover, they had been the progenitors of these orders, they would almost
certainly have been long ago supplanted and exterminated by their numerous
and improved descendants.

Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long as, or
probably far longer than, the whole interval from the Silurian age to the
present day; and that during these vast, yet quite unknown, periods of
time, the world swarmed with living creatures. {308}

To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most eminent
geologists, with Sir E. Murchison at their head, are convinced that we see
in the organic remains of the lowest Silurian stratum the dawn of life on
this planet. Other highly competent judges, as Lyell and the late E.
Forbes, dispute this conclusion. We should not forget that only a small
portion of the world is known with accuracy. M. Barrande has lately added
another and lower stage to the Silurian system, abounding with new and
peculiar species. Traces of life have been detected in the Longmynd beds,
beneath Barrande's so-called primordial zone. The presence of phosphatic
nodules and bituminous matter in some of the lowest azoic rocks, probably
indicates the former existence of life at these periods. But the difficulty
of understanding the absence of vast piles of fossiliferous strata, which
on my theory no doubt were somewhere accumulated before the Silurian epoch,
is very great. If these most ancient beds had been wholly worn away by
denudation, or obliterated by metamorphic action, we ought to find only
small remnants of the formations next succeeding them in age, and these
ought to be very generally in a metamorphosed condition. But the
descriptions which we now possess of the Silurian deposits over immense
territor