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 intercrossing, on the rate of breeding, on the
slowly changing physical conditions of the country, and more
especially on the nature of the other inhabitants with which the
varying species comes into competition.
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