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.
Pages:
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549