258 PROCEEDINGS OF THE AMERICAN ACADEMY. 
ants of the second seed would at once be subjected to the same modifying 
influences that had affected those of the first. 
It is thus clear that the insular form, having once started its diver- 
gence from the continental type, would be likely to differentiate itself 
more and more and not be much affected by the occasional arrival of 
scattered and isolated seeds from the continent. Furthermore, if seeds 
of the Chatham form were by any chance carried to other islands of the 
archipelago, further specialized races would be almest certain to arise 
bearing much the same relation to that of Chatham that it bore to the 
continental form, and quite as little affected by subsequent seedings. 
Thus unchecked, the races would have every opportunity to develop into 
more and more highly differentiated forms, varieties, and ultimately into 
well-marked species characteristic of particular islands. 
This may all seem purely hypothetical, but it will be seen that the 
conclusions rest upon only two very natural premises, namely, that seed- 
transference between the mainland and the islands or between the islands 
themselves, does not occur in the case of particular plants oftener, on the 
average, than once in several years, and in the second place that plants 
have multiplied on the islands as rapidly as they have frequently been 
observed to multiply elsewhere. When these two not unreasonable 
postulates are admitted, it is clearly no harder to account for the exist- 
ence of a “ harmonic”’ flora on islands of emergence than of subsidence. 
Indeed, it is what is to be expected in an archipelago of either kind 
where seed-transference is relatively rare. That this condition obtains 
in the case of the Galapagos is clearly shown by the existing diversity in 
the floras of the different islands, — a condition which could not continue 
if seed-transference were very common between the islands. The fact 
that it is not more frequent is perhaps sufficiently explained by the arid 
and sterile shores, which would certainly offer to most seeds washed 
thither by oceanic currents an exceedingly poor chance of surviving. 
The great existing difference between the Galapageian flora as a 
whole and that of tropical America is doubtless due not only to the 
differentiation of the insular forms, but also and perhaps in a consider- 
able degree to changes which have been simultaneously going on in the 
continental flora itself. Thus the ancestral forms of many Galapageian 
plants, for instance of the above mentioned Huphorbia viminea (which 
might reasonably be sought in the deserts of Peru), have probably 
failed to persist at all upon the mainland. ‘The fact that the florulae of 
the several islands are on the whole much more like each other than 
any one of them is like any part of the continental flora is perhaps 
