666 The American Naturalist. [August, 
careful examination shows that these are not mere individual 
differences, due to chance, state of development, or individual 
environment, but each form appears in general to be restricted 
to a single island. Some forms, such as those from Abingdon 
and Tower Islands, differ rather strikingly from the rest, while 
others present slighter differences, in a few cases too slight, 
that a series of careful measurements is necessary to demon- 
strate their existence. But the examination of considerable 
number of specimens, such as those secured by Dr. Baur, shows 
that the species as it occurs on each island differs in some 
characteristics, slight or more considerable from the forms of 
all or nearly all the other islands, and furthermore, each island 
appears to have only one form of its own. 
The question at once presents itgelf: 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 of 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 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 there spread to the rest. That the same 
species should have reached all these islands presupposes pe 
considerable 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 rela- 
tive or complete 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 
