EUPHORBIA AND ITS ORIGIN 315 



come about after the introduction of the species on the islands. 

 Continuing the supposition that these are islands of elevation, 

 the seeds of Euphorbia viminea must have reached them in 

 one or two ways. Either every one 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 have 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 develop- 

 ment of the forms on the different islands. For relative or 

 complete isolation seems necessary to account for the racially 

 divergent floras of the islands, and especially for the occur- 

 rence of only one form on each island. On Dr. Baur's 

 assumption of a former union between the islands, and sub- 

 sequent separation by subsidence, the authors maintain that 

 not only is an explanation of the facts possible, but the exist- 

 ing flora of the archipelago is just that which would most 

 naturally result from such an origin. A former union of the 

 islands would account at once for the occurrence of identical 

 ancestral species upon the different members of the group. 

 The subsequent separation would give the needed isolation for 

 varietal and racial divergence, while the latter could not have 

 come about if a continual interchange of seed were taking 

 place from island to island. 



Messrs. Robinson and Greenman's careful reasoning is 

 just as well applicable to the birds of the genera Geospiza, 

 Certhidea and Nesomimus, to the reptilian genera Tropidurus 

 and Testudo, and to the snail Bulimulus as it is to Euphorbia 

 viminea, and from a study of any of them we should come to 

 precisely the same conclusion as these authors. Mr. 

 Hemsley * thinks the biological data which we possess from 

 the Galapagos islands are strongly in favour of Professor 

 Baur's views, and he supposes the area on which the islands 

 stand to have been continued eastward to the mainland of 

 Veraguas. 



Professor Stewart does not produce any new data for or 



* Hemsley, W. Botting, " Insular Floras," VI. (a), p. 299. 



