GEOGRAPHICAL DISTRIBUTION 361 



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 inter- 

 crossing with them. With respect to the effects of this 

 intercrossing, 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 1 

 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 habits, and will consequently 

 have been little liable to modification. Any tendency 

 to modification will, also, have been checked by inter- 

 crossing with the unmodified immigrants from the 

 mother-country. Madeira, again, is inhabited by a 

 wonderful number of peculiar land-shells, whereas not 

 one species of sea-shell is confined to its shores : now, 

 though we do not know how sea-shells are dispersed, yet 



