ely,
that it is a case of temporary migration from a distinct geographical
province, seems satisfactory.

These several facts accord well with our theory, which includes no fixed
law of development, causing all the inhabitants of an area to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be slow, and will generally affect only a few species
at the same time; for the variability of each species is independent of
that of all others. Whether such variations or individual differences as
may arise will be accumulated through natural selection in a greater
or less degree, thus causing a greater or less amount of permanent
modification, will depend on many complex contingencies--on
the variations being of a beneficial nature, on the freedom of
intercrossing, on the slowly changing physical conditions of the
country, on the immigration of new colonists, and on the nature of the
other inhabitants with which the varying species come 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, should
change in a less degree. We find similar relations between the existing
inhabitants of distinct countries; for instance, the land-shells and
coleopterous insects of Madeira have 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 any area have become modified and improved,
we can understand, on the principle of competition, and from the
all-important relations of organism to organism in the struggle for
life, that any form which did not become in some degree modified and
improved, would be liable to extermination. Hence, we see why all
the species in the same region do at last, if we look to long enough
intervals of time, become modified; for otherwise they would become
extinct.

In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of enduring formations, rich in fossils, depends on gre