188 THE ORIGIA' OF SPECIES 



Although in oceanic islands the species are few in 

 number, the proportion of endemic kinds {i.e., those 

 found nowhere else in tlie world) is often extremely 

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

 demic 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 \o compete with new associates, would be emi- 

 nently 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-countrv, with which the insular forms 

 have intercrossed. It should be borne in mind that the 

 offspring of such crosses would certainly gain in vigor; 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 



