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. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or, if
changing, that it should change less. We see the same fact in
geographical distribution; for instance, in the land-shells and
coleopterous insects of Madeira having come to differ considerably from
their nearest allies on the continent of Europe, whereas the marine
shells and birds have remained unaltered. We can perhaps understand the
apparently quicker rate of change in terrestrial and in more highly
organised productions compared with marine and lower productions, by
the more complex relations of the higher beings to their organic and
inorganic conditions of life, as explained in a former chapter. When
many of the inhabitants of a country have become modified and improved,
we can understand, on the principle of competition, and on that of the
many all-important relations of organism to organism, that any form
which does not become in some degree modified and improved,
will be liable to be exterminated. Hence we can see why all the species
in the same region do at last, if we look to wide enough intervals of
time, become modified; for those which do not change will become
extinct.
237