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.
Pages:
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647