136 Robinson and Greenman—Galapagos Flora. 
formsthan Huphorbia viminea Hook. fil. This species differs 
markedly in foliage from any other known member of this 
large genus, and is characteristic of the Galapagos Archipelago. 
Being essentially a desert plant, it can subsist even upon those 
islands of the group which are of low altitude and do not 
attain the upper regions of moister atmosphere. It was first 
collected by Macrze upon Albemarle, rediscovered by Anders- 
son on Charles Island, and has now been collected by Dr. 
Baur upon the following islands: Barrington, Chatham, South- 
ern and Eastern Albemarle, James, Jervis, Bindloe, Tower, and 
Abingdon... Even the most cursory inspection of the forms from 
these different islands discloses marked variation in the contour, 
size, thickness, rigidity, and color of the leaves, as well as in 
the length of the internodes, color of the stems, ete., while 
more careful examination shows that these are not mere indi- 
vidual differences, due to chance, state of development, or indi- 
vidual environment, but each form appears in general to be 
restricted to a single island. Some forms, such as those of 
Abingdon and Tower islands, differ rather strikingly from the 
rest, while others present slighter differences; in a few cases 
so slight, that a series of careful measurements is necessary to 
demonstrate their existence. But the examination of a con- 
siderable number of specimens, such as those secured by Dr. 
Baur, shows that the species, as it occurs upon each island, 
differs in some characteristics, slight or more considerable, from 
the forms of all or nearly all of the other islands, and further- 
more each island appears to have only one form of its own. 
The question at once presents itself, if this archipelago is 
composed of islands of elevation, built up from the sea-floor 
independently by volcanic action, how has such a distribution 
been effected. If the vegetation has been derived from the 
mainland by the chance transportation of seeds, it is quite 
impossible to believe that each island has received a slightly 
different form of the same species, and we are forced to the 
much more natural assumption that racial and varietal diver- 
gence has come about after the introduction of the species 
upon the islands. Now, continuing the supposition that these 
are islands of elevation, the seeds of Huphorbia 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 con- 
siderable facility of transportation. But as soon as this is 
granted it is impossible to understand the highly individual 
development of the forms upon the different islands. For rel- 
