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.
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