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 
wo 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. 
