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, etc., 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.

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 R. 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 territories in
Russia and in North America, do not support the view, that the older a
formation is, the more it has suffered the extremity