pete with new
associates, would be eminently liable to modification, and would 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 partly on the species
which are not modified having immigrated in a body, so that their mutual
relations have not been much disturbed; and partly on the frequent
arrival of unmodified immigrants from the mother-country, with which
the insular forms have intercrossed. It should be borne in mind that the
offspring of such crosses would certainly gain in vigour; so that even
an occasional cross would produce more effect than might have been
anticipated. I will give a few illustrations of the foregoing remarks:
in the Galapagos Islands there are twenty-six land birds; of these
twenty-one (or perhaps twenty-three) are peculiar; whereas of the eleven
marine birds only two are peculiar; and it is obvious that marine birds
could arrive at these islands much more easily and frequently 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 a single
endemic land bird; and we know from Mr. J.M. Jones's admirable account
of Bermuda, that very many North American birds occasionally or even
frequently visit this island. Almost every year, as I am informed by
Mr. E.V. Harcourt, many European and African birds are blown to Madeira;
this island is inhabited by ninety-nine kinds, of which one alone is
peculiar, though very closely related to a European form; and three or
four other species are confined to this island and to the Canaries.
So that the islands of Bermuda and Madeira have been stocked from the
neighbouring continents with birds, which for long ages have there
struggled together, and have become mutually co-adapted. Hence, when
settled in their new homes, each kind will have been kept by the others
to its proper place and habits, and will consequently have been but
little liable to modification. Any tendency to modification will also
have been checked by intercrossing with the unmodified immigrants,
often arriving from the mother-country. Madeira again is inhabited by
a wonderful number of peculiar land-shells, whereas not one species
of sea-she