n of organic beings accord best with the common view
of the immutability of species, or with that of their slow and gradual
modification, through variation and natural selection.

New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between the stages,
and to make the proportion between the lost and existing forms more
gradual. In some of the most recent beds, though undoubtedly of high
antiquity if measured by years, only one or two species are extinct,
and only one or two are new, having appeared there for the first time,
either locally, or, as far as we know, on the face of the earth. The
secondary formations are more broken; but, as Bronn has remarked,
neither the appearance nor disappearance of the many species embedded in
each formation has been simultaneous.

Species belonging to different genera and classes have not changed at
the same rate, or in the same degree. In the 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 an
existing crocodile is associated with many 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 at a quicker rate than those of the
sea, of which a striking instance has been observed in Switzerland.
There is some reason to believe that organisms high in the scale, 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 not
the same in each successive so-called formation. 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 no reason to believe that the same
identical form ever reappears. The strongest apparent exception to this
latter 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 Lyell's explanation, nam