PDF - Wallace Online

34 ZOOLOGICAL GEOGRAPHY. [part hi.
immigrants. We know that small Passerine birds annually reach
the Bermudas from America, and the Azores from Europe, the
former travelling over 600, the latter over 1000 miles of ocean.
These groups of islands are both situated in stormy seas, and the
immigrants are so numerous that hardly any specific change in the
resident birds has taken place. The Galapagos receive no such
annual visitants ;
hence, when by some rare accident a few indi-
viduals of a species did arrive, they remained isolated, probably
for thousands of generations, and became gradually modified
through natural selection under completely new conditions of
existence. Less rare and violent storms would suffice to carry
some of these to other islands, and thus the archipelago would
in time become stocked. It would appear probable, that those
which have undergone most change were the earliest to arrive
so that we might look upon the three peculiar genera of finches,
and Certhidea, the peculiar form of Coerebidae, as among the most
ancient inhabitants of the islands, since they have become so
modified as to have apparently no near allies on the mainland.
But other birds may have arrived nearly at the same time, and
yet not have been much changed. A species of very wide
range, already adapted to live under very varied conditions and
to compete with varied forms of life, might not need to become
modified so much as a bird of more restricted range, and more
specialized constitution. And if, before any considerable change
had been effected, a second immigration of the same species
occurred, crossing the breed would tend to bring back the original
type of form. While, therefore, we may be sure that birds like
the finches, which are profoundly modified and adapted to the
special conditions of the climate and vegetation, are among the
most ancient of the colonists ; we cannot be sure that the less
modified form of tyrant-flycatcher or mocking-thrush, or even
the unchanged but cosmopolitan owl, were not of coeval date
since even if the parent form on the continent has been changed,
successive immigrations may have communicated the same
change to the colonists.
The reptiles are somewhat more difficult to account for. We
know, however, that lizards have some means of dispersal over