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1859
1860
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1866
1869
1872

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1860
1861
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1872

but, as they are rare, they may be here passed over. 1859 1860 1861
but, as they are not common, they may be here passed over. 1866
thus, Trautschold gives a number of instances with Ammonites; and Hilgendorf has described a most curious case of ten graduated forms of Planorbis multiformis in the successive beds of a fresh-water formation in Switzerland. 1869 1872

I can see 1859 1860 1861 1866
OMIT 1869 1872

why 1859 1860 1861 1866
can be given why 1869 1872

but I can by no means pretend to 1859 1860 1861
at its commencement and close; but I cannot 1866 1869 1872

With 1859 1860 1861 1866 1869
We may safely infer that with 1872

we may safely infer 1859 1860 1861 1866 1869
there has been 1872

why we do not therein find closely graduated varieties between the allied species which lived at its commencement and at its close.
Several
Some
cases are on record of the same species presenting
distinct
distinct
varieties in the upper and lower parts of the same
formation:
formation,
but, as they are rare, they may be here passed over. Although each formation has indisputably required a vast number of years for its deposition, I can see several reasons why each should not
commonly include
include
a graduated series of links between the species which
then
then
lived
lived;
but I can by no means pretend to assign due proportional weight to the following considerations.
Although each formation may mark a very long lapse of years, each
probably
perhaps
is short compared with the period requisite to change one species into another. I am aware that two palæontologists, whose opinions are worthy of much deference, namely Bronn and
Wood-ward,
Wood- ward,
Woodward,
have concluded that the average duration of each formation is twice or thrice as long as the average duration of specific forms. But insuperable difficulties, as it seems to me, prevent us
from coming
coming
to any just conclusion on this head. When we see a species first appearing in the middle of any formation, it would be rash in the extreme to infer that it had not elsewhere previously existed. So again when we find a species disappearing before the
last
uppermost
layers have been deposited, it would be equally rash to suppose that it then became
wholly
wholly
extinct. We forget how small the area of Europe is compared with the rest of the world; nor have the several stages of the same formation throughout Europe been correlated with perfect accuracy.
With marine animals of all
kinds
kinds,
we may safely infer a large amount of migration
due to
during
climatal and other changes; and when we see a species first appearing in any formation, the probability is that it