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first bed of the Silurian 1859 1860 1861 1866
Cambrian 1869 1872

I 1859 1860 1861
We now know that animals, and probably plants, lived at an epoch immensely remote, long anterior to the primordial zone of the Silurian system, but I 1866
We now know that at least one animal did then exist; but I 1869 1872

as far as we can see, 1859 1860 1861
OMIT 1866 1869 1872

for an enormous period extended, 1859 1860 1861
extended for an enormous period, 1866 1869 1872

Silurian epoch; 1859 1860 1861
commencement of the Silurian epoch; 1866
commencement of the Cambrian system; 1869 1872

all be 1859 1860 1861
only as remnants 1866 1869 1872

seem to me simply to follow on 1859 1860 1861 1866 1869
agree admirably with 1872

natural 1859 1860 1861 1866 1869
variation and natural 1872

of the larger 1859 1860 1861 1866
belonging to large and 1869 1872

may overlook how important a part migration
has
must have
played, when the formations of any one great
region,
region
alone,
alone,
as
those
that
of Europe, are considered; he may urge the apparent, but often falsely apparent, sudden coming in of whole groups of species. He may ask where are the remains of those infinitely numerous organisms which must have existed long before the first bed of the Silurian system was
deposited?
deposited:
I can answer
the above
this last
this latter
question only
hypothetically,
hypothetically,
by
supposing
saying
that as far as we can see, where our oceans now extend they have for an enormous period extended, and where our oscillating continents now stand they have stood
ever
ever
since the Silurian epoch; but
that,
that
long before that
epoch,
period,
the world
may have
may have
presented a
widely
wholly
different aspect; and that the older continents, formed of formations older than any known to us,
exist
may
now all be in a metamorphosed condition, or
may
may
lie
wholly buried
still buried
buried
under the ocean.
Passing from these difficulties,
all
all
the other great leading facts in palæontology seem to me simply to follow on the theory of descent with modification through natural selection. We can thus understand how it is that new species come in slowly and successively; how species of different classes do not
necessarily,
necessarily
change together, or at the same rate, or in the same degree; yet in the long run that all undergo modification to some extent. The extinction of old forms is the almost inevitable consequence of the production of new forms. We can understand why when a species has once disappeared it never reappears. Groups of species increase in numbers slowly, and endure for unequal periods of time; for the process of modification is necessarily slow, and depends on many complex contingencies. The dominant species of the larger dominant groups tend to leave many modified