256 PROCEEDINGS OP THE AMERICAN ACADEMY. 



the supposition that these are islands of elevation, the seeds of Euphorbia 

 viminea must have reached them in one of two ways : either each of the 

 nine islands, where we know the species now to occur, must have received 

 its seed directly from the mainland, or, what is much more natural, seed 

 must have reached one or more of the islands and from these spread to 

 the rest. That the same species should have reached all these islands 

 presupposes a considerable facility of transportation. But as soon as 

 this is granted it is impossible to understand the highly individual de- 

 velopment of the forms upon the different islands. For relative or com- 

 plete isolation seems necessary to account for the racially divergent floras 

 of the islands ; and especially for the occurrence of only one form upon 

 each island. It would thus appear necessary, in accounting for the 

 present distribution, to assume that at one time in the remote past, the 

 islands were either united, or at least that the channels which separate 

 them were less formidable barriers to seed-transportation than at present, 

 so that a general distribution of species could have been effected ; and 

 that subsequently, as the islands separated, or as the channels through 

 some change of currents, or other cause, became less easily passed, au 

 era of much greater isolation of the floras of the different islands came 

 about. The divergence of character of the vegetation would then begin 

 at once, and the otherwise unaccountable existence of a single and 

 peculiar form upon each island would be readily intelligible. While not 

 prepared to make any positive assertion regarding the pi'obable origin of the 

 islands, the authors fail to see in the hitherto generally accepted theory 

 of elevation any satisfactory explanation for the harmonic yet divergent 

 floras of the different members of the group." 



Finally, for the subsidence theory, it must be admitted that the direct 

 geological arguments for the elevation of the islands are not so forcible 

 as they at first appear. Thus, as Baur has pointed out, the fact that all 

 parts of the islands now visible are volcanic proves little ; for if the 

 Andes were sunk until only equivalent land areas remained, they too 

 would appear wholly volcanic ; and as to the recent elevation of the 

 South American coast, that, as I am informed by Professor W. M. Davis, 

 is no conclusive proof that areas five hundred miles to the seaward 

 have suffered like elevation or, indeed, that they have not been simulta- 

 neously subjected to a sort of counter-balancing subsidence. 



Such, in brief, have been the arguments advanced on both sides re- 

 garding the origin of the Galapagos islands. During a re-examination 

 of the whole vascular flora of the islands, I have sought further light 

 upon this question, and now tiud the peculiar distribution of the plants 



