er 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,
{315} 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.

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 long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and irregularly
intermittent intervals; consequently the amount of organic change exhibited
by the fossils embedded in consecutive formations is not equal. Each
formation, on this view, does not mark a new and complete act of creation,
but only an occasional scene, taken almost at hazard, in a slowly changing
drama.

We can clearly understand why a species when once lost should never
reappear, even if the very same condi