The naturalist must be dull who is not led to inquire
what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we see in
the case of varieties, nearly alike. The dissimilarity of the inhabitants
of different regions may be attributed to modification through variation
and natural selection, and probably in a subordinate degree to the definite
influence of different physical conditions. The degrees of dissimilarity
will depend on the migration of the more dominant forms of life from one
region into another having been more or less effectually prevented, at
periods more or less remote--on the nature and number of the former
immigrants--and on the action of the inhabitants on each other in leading
to the preservation of different modifications; the relation of organism to
organism in the struggle for life being, as I have already often remarked,
the most important of all relations. Thus the high importance of barriers
comes into play by checking migration; as does time for the slow process of
modification through natural selection. Widely-ranging species, abounding
in individuals, which have already triumphed over many competitors in their
own widely-extended homes, will have the best chance of seizing on new
places, when they spread out into new countries. In their new homes they
will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants.
Pages:
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596