Sicily,
the successive changes in the marine inhabitants of that island have been
many and most gradual. The secondary formations are more broken; but, as
Bronn has remarked, neither the appearance {313} nor disappearance of their
many now extinct species has been simultaneous in each separate formation.

Species of different genera and classes have not changed at the same rate,
or in the same degree. In the oldest 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, in an existing crocodile
associated with many strange and 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 change at a quicker rate than those of the sea, of which a striking
instance has lately been observed in Switzerland. There is some reason to
believe that organisms, considered high in the scale 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, 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 to believe that the same identical form never
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, namely, that it is a case of temporary
migration from a distinct geographical province, seems to me satisfactory.
{314}

These several facts accord well with my theory. I believe in no fixed law
of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species is
quite independent of that of all others. Whether such variability be taken
advantage of by natural selection, and wheth