CHAP. XUI.J INHABITANTS OF OCEANIC ISLANDS. 327 



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

 the Galapagos Archipelago, with the number found on any conti- 

 nent, 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 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 99 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 immi- 

 grants, often arriving from the mother-country. Madeira again is 

 inhabited by a wonderful number of peculiar land-shells, whereas 

 not one species of sea-shell is peculiar to its shores : now, though 

 we do not know how sea-shells are dispersed, yet we can see that 



