United States; and from the revolution in our pal ontological ideas on
many points, which the discoveries of even the last dozen years have
effected, it seems to me to be about as rash in us to dogmatize on the
succession of organic beings throughout the world, as it would be for a
naturalist to land for five minutes on some one barren point in
Australia, and then to discuss the number and range of its productions.
_On the sudden appearance of groups of Allied Species in the lowest
known fossiliferous strata_. There is another and allied difficulty,
which is much graver. I allude to the manner in which numbers of
species of the same group, suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group have descended from one
progenitor, apply with nearly equal force to the earliest known
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, 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
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