Chap. XIIL] INHABITANTS OF OCEANIC ISLANDS. 179 



found nowhere else in the world) is often extremely 

 large. If we compare, for instance, the number of 

 endemic land-shells in Madeira, or of endemic birds in 

 the Galapagos Archipelago, with the number found on 

 any continent, and then compare the area of the island 

 with that of the continent, we shall see that this is 

 true. This fact might have been theoretically expected, 

 for, as already explained, species occasionally arriving 

 after long intervals of time in the new and isolated 

 district, and having to compete 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 

 immigated 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 26 land-birds ; of these 

 21 (or perhaps 23) are peculiar, whereas of the 11 

 marine birds only 2 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 



