h cannot cross
wide spaces of ocean, as frogs and terrestrial mammals, should not inhabit
oceanic islands; and why, on the other hand, new and peculiar species of
bats, which can traverse the ocean, should so often be found on islands far
distant from any continent. Such facts {478} as the presence of peculiar
species of bats, and the absence of all other mammals, on oceanic islands,
are utterly inexplicable on the theory of independent acts of creation.

The existence of closely allied or representative species in any two areas,
implies, on the theory of descent with modification, that the same parents
formerly inhabited both areas; and we almost invariably find that wherever
many closely allied species inhabit two areas, some identical species
common to both still exist. Wherever many closely allied yet distinct
species occur, many doubtful forms and varieties of the same species
likewise occur. It is a rule of high generality that the inhabitants of
each area are related to the inhabitants of the nearest source whence
immigrants might have been derived. We see this in nearly all the plants
and animals of the Galapagos archipelago, of 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 {479}
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