OCEANIC ISLANDS, 195 



nent. Although they are related to continental 

 species, they are all of them distinct from any found 

 on the mainland, some of them being so distinct as 

 to be sometimes ranked as different genera. The 

 two snakes are very different from each other, 

 belonging to two families, and it is necessary, there- 

 fore, to assume that two immigrations of snakes 

 have occurred in the past to account for the present 

 species. The tortoises, although related to those 

 on the neighboring continent, are distinct species, 

 and are very remarkable animals, having developed 

 to an enormous size, doubtless from the lack of any 

 dangerous enemies. Of the fifty-seven species of 

 birds thirty-eight are peculiar to the islands, and of 

 the other nineteen all but one belong to the water- 

 birds, which are always great wanderers ; and seven 

 of the water-birds are peculiar. The only land bird 

 common to the islands and the continent is the 

 American rice bird (bobolink), which ranges over the 

 whole of North and part of South America. Only 

 a few insects have been found, most of which are 

 peculiar to the island. And the same may be said 

 of the Mollusca. Of three hundred and thirty-two 

 known species of flowering plants one hundred and 

 seventy-four are peculiar, and about twenty more 

 have been introduced by man. The rest are South 

 American species. Considering, therefore, the close 

 relation of all these species to South American 

 forms, it cannot be doubted that they were origi- 

 nally derived from that continent ; and considering 

 that they are very distinct species from any on the 

 continent, it is hardly possible to doubt that they 



