Juan Fernandez, and of the other American islands being related in the
most striking manner to the plants and animals of the neighbouring
American mainland; and those of the Cape de Verde archipelago and other
African islands to the African mainland. It must be admitted that these
facts receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group,
and with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies
of extinction and divergence of character. On these same principles we
see how it is, that the mutual affinities of the species and genera
within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification; why
adaptive characters, though of paramount importance to the being, are
of hardly any
importance in classification; why characters derived from rudimentary
parts, though of no service to the being, are often of high
classificatory value; and why embryological characters are the most
valuable of all. The real affinities of all organic beings are due to
inheritance or community of descent. The natural system is a
genealogical arrangement, in which we have to discover the lines of
descent by the most permanent characters, however slight their vital
importance may be.
The framework of bones being the same in the hand of a man, wing of a
bat, fin of the porpoise, and leg of the horse, the same number of
vertebr forming the neck of the giraffe and of the elephant, and
innumerable other such facts, at once explain themselves on the theory
of descent with slow and slight successive modifications. The
similarity of pattern in the wing and leg of a bat, though used for
such different purpose, in the jaws and legs of a crab, in the petals,
stamens, and pistils of a flower, is likewise intelligible on the view
of the gradual modification of parts or organs, which were alike in the
early progenitor of each class. On the principle of successive
variations not always supervening at an early age, and being inherited
at a corresponding not early period of life, we can clearly see why the
embryos of mammals, birds, reptiles, and fishes should be so closely
alike, and should be so unlike the adult forms. We may cease marvelling
at the embryo of an air-breathing mammal or bird having branchial slits
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