rossing, 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.

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 conditions of life, organic and
inorganic, should recur. For though the offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances) to
fill the exact place of another species in the economy of nature, and
thus supplant it; yet