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, 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.
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