° 
FLORA OF THE GALAPAGOS ISLANDS. 257 
less difficult to account for on the emergence theory than it seemed 
when the Baur plants were studied some years ago. 
Let us consider theoretically what would be likely to happen to a 
plant casually introduced in a pelagic archipelago, where no plant of its 
particular affinity had previously found its way. Let us suppose, for 
instance, that the ancestral form of Zuphorbia viminea reached Chatham 
Island from the continent, and that conditions of seed-transportation 
were such that subsequent seedings from the mainland would not be 
likely to happen oftener on the average than once in thirty, fifty, or 
perhaps one hundred years, —no unreasonable assumption. Now, it is 
known from observation that a plant introduced into a new region can 
overrun considerable territory and increase to thousands of individuals, 
even in a shorter space of time. It is further likely that a plant estab- 
lished upon such an island would be at once exposed to modifying 
influences and tend toward the formation of a new race particularly 
suited to its altered environment. Whether this were effected by direct 
Influence or by natural selection is not significant in this case. In the 
interval between the first and second seeding the change would probably 
be very slight and taxonomically imperceptible, but that some modifica- 
tion would have taken place seems likely. It is clear that the second 
and subsequent seedings of the same island by the same species from 
the continent would tend by the infusion of pure stock to reclaim the 
incipient insular variation to the typical continental form of the species. 
But to see how great or rather how slight the influence of these later 
seedings would be, it is only necessary to consider the numerical relation 
of both forms, The descendants of the first immigrant might well have 
increased to many thousands of (slightly altered) individuals before the 
second seed arrived from the mainland, Thus the reclaiming influence 
of this second immigrant would not be as one to one but as one against 
thousands, that is to say, virtually nil. Of course, it might be thought 
that as the first seed increased in a few years to thousands of individuals, 
the second might do so likewise, so that their influence would after all 
be not very unequal. This, however, could scarcely happen for aud 
reasons. In the first place the original seed would have found in the 
island (before uninhabited by any near relative) a fresh terrain, giving 
it an opportunity to multiply rapidly. The second immigrant, however, 
would find its proper habitat on the island no longer tres for settlement, 
but largely if not entirely occupied by countless individuals of a very 
nearly related stock. It eould scarcely fail to cross repeatedly with = 
insular form and quickly merge into it, the more S0 because the d 
vou, xxxvu1.—17 
