278 
ISLAND LIFE. 
[part II. 
various epochs by arms of the sea uniting the two oceans across 
what is now Central America (the last separation being of recent 
date, as shown by the identical species of fishes on both sides 
of the isthmus), and the influence of the glacial epoch in driving 
the temperate American flora southward along the mountain 
plateaus . 1 At the time when the two oceans were united a 
portion of the Gulf Stream may have been diverted into the 
Pacific, giving rise to a current, some part of which would 
almost certainly have reached the Galapagos, and this may have 
helped to bring about that singular assemblage of West Indian 
and Mexican plants now found there. And as we now believe 
that the duration of the last glacial epoch in its successive 
phases was much longer than the time which has elapsed since 
it finally passed away, while throughout the Miocene epoch the 
snow-line would often be lowered during periods of high ex- 
centricity, we are enabled to comprehend the nature of the 
causes which may have led to the islands being stocked 
with those northern or sub-alpine types which are so char- 
acteristic a feature of that portion of the Galapagos flora 
which consists of peculiar species. 
On the whole, the flora agrees with the fauna in indicating 
a moderately remote origin, great isolation, and changes of con- 
ditions affording facilities for the introduction of organisms from 
various parts of the American coast, and even from the West 
Indian Islands and Gulf of Mexico. As in the case of the birds, 
the several islands differ considerably in their native plants, many 
species being limited to one or two islands only, while others 
extend to several. This is, of course, what might be expected 
on any theory of their origin ; because, even if the whole of the 
islands had once been united and afterwards separated, long 
continued isolation would often lead to the differentiation of 
species, while the varied conditions to be found upon islands 
differing in size and altitude as well as in luxuriance of vegeta- 
tion, would often lead to the extinction of a species on one island 
and its preservation on another. If the several islands had been 
equally well explored, it might be interesting to see whether, as 
in the case of the Azores, the number of species diminished in 
1 Geographical Distribution of Animals , Vol. II. p. 81. 
