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now extinct species 1859 1860 1861 1866
extinct species 1869
species embedded in each formation 1872

has been simultaneous 1859 1860 1861 1866
embedded 1869
has been simultaneous. 1872

formation. 1859 1860 1861 1866
formation has been simultaneous. 1869
OMIT 1872

strictly correspond with the succession of our geological formations; so that between each two consecutive formations, the forms of life have seldom changed in exactly 1859 1860 1861 1866
OMIT 1869 1872

degree. 1859 1860 1861 1866
in each successive so-called formation. 1869 1872

nor disappearance of
the
their
many now extinct species has been simultaneous
in
in
each
each
separate
separate
separate
formation.
Species
belonging to
of
different genera and classes have not changed at the same rate, or in the same degree. In the
oldest
older
tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms. Falconer has given a striking instance of a similar fact,
for
in
an existing crocodile
is associated
associated
with many
strange and
strange
strange
lost mammals and reptiles in the sub-Himalayan deposits. The Silurian Lingula differs but little from the living species of this genus; whereas most of the other Silurian Molluscs and all the Crustaceans have changed greatly. The productions of the land seem to
have changed
change
at a quicker rate than those of the sea, of which a striking instance has
lately
lately
been observed in Switzerland. There is some reason to believe that
organisms
organisms,
considered
considered
high in the
scale,
scale
of nature,
of nature,
change more quickly than those that are low: though there are exceptions to this rule. The amount of organic change, as Pictet has remarked,
is
does
not strictly correspond with the succession of our geological formations; so that between each two consecutive formations, the forms of life have seldom changed in exactly the same degree. Yet if we compare any but the most closely related formations, all the species will be found to have undergone some change. When a species has once disappeared from the face of the earth, we have
reason
no reason
to believe that the same identical form
never
ever
reappears. The strongest apparent exception to this latter
rule
rule,
is that of the so-called "colonies" of M. Barrande, which intrude for a period in the midst of an older formation, and then allow the pre-existing fauna to reappear; but
Lyells
Lyell's
explanation, namely, that it is a case of temporary migration from a distinct geographical province, seems
to me
to me
satisfactory.