instance, the number of the endemic land-shells in Madeira, or of the
endemic birds in the Galapagos Archipelago, with the number found on
any continent, and then compare the area of the islands with that of
the continent, we shall see that this is true. This fact might have
been expected on my theory, for, as already explained, species
occasionally arriving after long intervals in a new and isolated
district, and having to compete with new associates, will be eminently
liable to modification, and will often produce groups of modified
descendants. But it by no means follows, that, because in an island
nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend on the species which do not become modified
having immigrated with facility and in a body, so that their mutual
relations have not been much disturbed. Thus in the Galapagos Islands
nearly every land-bird, but only two out of the eleven marine birds,
are peculiar; and it is obvious that
marine birds could arrive at these islands more easily than land-birds.
Bermuda, on the other hand, which lies at about the same distance from
North America as the Galapagos Islands do from South America, and which
has a very peculiar soil, does not possess one endemic land bird; and
we know from Mr. J. M. Jones s admirable account of Bermuda, that very
many North American birds, during their great annual migrations, visit
either periodically or occasionally this island. Madeira does not
possess one peculiar bird, and many European and African birds are
almost every year blown there, as I am informed by Mr. E. V. Harcourt.
So that these two islands of Bermuda and Madeira have been stocked by
birds, which for long ages have struggled together in their former
homes, and have become mutually adapted to each other; and when settled
in their new homes, each kind will have been kept by the others to
their proper places and habits, and will consequently have been little
liable to modification. Madeira, again, is inhabited by a wonderful
number of peculiar land-shells, whereas not one species of sea-shell is
confined to its shores: now, though we do not know how seashells are
dispersed, yet we can see that their eggs or larv , perhaps attached to
seaweed or floating timber, or to the feet of wading-birds, might be
transported far more easily than land-shells, across three or four
hundred miles of open sea. The different orders of insects in Madeira
apparently present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and their
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