the
world) is often extremely large. If we compare, for 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 partly 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; and partly on the frequent
arrival of unmodified immigrants from the mother-country, and the
consequent intercrossing with them. With respect to the effects of this
intercrossing, {391} it should be remembered that the offspring of such
crosses would almost certainly gain in vigour; so that even an occasional
cross would produce more effect than might at first have been anticipated.
To give a few examples: 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 habit