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.

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 whether the variations be
accumulated to a greater or lesser amount, thus causing a greater or
lesser amount of modification in the varying species, depends on many
complex contingencies,--on the variability being of a beneficial nature,
on the power of interc