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stratum 1859 1860 1861 1866 1872
or Cambrian stratum 1869

yet quite unknown, periods of time, 1859 1860 1861 1866
periods 1869 1872

4 blocks not present in 1859 1860 1861 1866; present in 1869 1872
Here we encounter a formidable objection; for it seems doubtful whether the earth in a fit state for the habitation of living creatures has lasted long enough. Sir W. Thompson concludes that the consolidation of the crust can hardly have occurred less than 20 or more than 400 million years ago, but probably not less than 98 or more than 200 million years. These very wide limits show how doubtful the data are; and other elements may have to be introduced into the problem. Mr. Croll estimates that about 60 million years have elapsed since the Cambrian period, but this, judging from the small amount of organic change since the commencement of the Glacial epoch, seems a very short time for the many and great mutations of life, which have certainly occurred since the Cambrian formation; and the previous 140 million years can hardly be considered as sufficient for the development of the varied forms of life which certainly existed towards the close of the Cambrian period.

1 blocks not present in 1859 1860 1861 1866 1869; present in 1872
It is, however, probable, as Sir William Thompson insists, that the world at a very early period was subjected to more rapid and violent changes in its physical conditions than those now occurring; and such changes would have tended to induce changes at a corresponding rate in the organisms which then existed.

records of these vast primordial periods, 1859 1860 1861
richly fossiliferous records of these vast primordial periods, 1866
rich fossiliferous deposits belonging to these assumed earliest periods, 1869
rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, 1872

of the most 1859 1860 1861
OMIT 1866 1869 1872

are 1859 1860 1861
were until recently 1866 1869 1872

M. Barrande has lately 1859 1860 1861
M. Barrande not long ago 1866
Not long ago M. Barrande 1869
Not very long ago M. Barrande 1872

to the Silurian system, 1859 1860 1861 1866
OMIT 1869 1872

species. 1859 1860 1861 1866
species beneath the old Silurian system. 1869
species, beneath the then known Silurian system; and now, still lower down in the Lower Cambrian formation, Mr. Hicks has found in South Wales beds rich in trilobites, and containing various molluscs and annelids. 1872

life have 1859 1860 1861
several forms have 1866
several forms have also 1869

in the Longmynd beds 1859
in the Longmynd beds, 1860 1861 1866
OMIT 1869

zone. 1859 1860 1861 1866
zone in the Longmynd group, now divided into two stages, and constituting the Lower Cambrian system. 1869

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. 1859 1860 1861
now within the last year the great discovery of the Eozoon in the Laurentian formation of Canada has been made; and after reading Dr. Carpenter's description of this remarkable fossil, it is impossible to feel any doubt regarding its organic nature. 1866

If these 1859 1860 1861
If the 1866
It does not seem probable that the 1869 1872

obliterated 1859 1860 1861
if their fossils had been wholly obliterated 1866
that their fossils have been wholly obliterated 1869 1872

we ought to find 1859 1860 1861
we ought to have found 1866
for if this had been the case we should have found 1869 1872

ought to be very generally 1859 1860 1861
ought to have existed almost always 1866
would always have existed 1869 1872

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
the
my
theory be true, it is indisputable
that,
that
before the lowest
Cambrian
Silurian
stratum was
deposited
deposited,
long periods elapsed, as long as, or probably far longer than, the whole interval from the
Cambrian
Silurian
age to the present day; and that during these
vast
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
beheld
see
in the organic remains of the lowest Silurian stratum the
first dawn
dawn
of
life.
life
on
on
this
this
planet.
planet.
Other highly competent judges, as Lyell and
the late
the late
E. Forbes ,
disputed
have disputed
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,
stage
to the Silurian system, abounding with new and peculiar species.
Remnants
Traces
of life have been detected in the Longmynd beds beneath
Barrandes
Barrande's
so-called primordial zone. The presence
also of
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
have
had
been
generally
quite
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