are to be trusted, it must be admitted that scarcely a single island
exists which has not recently been united to some continent. This view
cuts the Gordian knot of the dispersal of the same species to the most
distant points, and removes many a difficulty: but to the best of my
judgment we are not authorized in admitting such enormous geographical
changes within the period of existing species. It seems to me that we
have abundant evidence of great oscillations of level in our
continents; but not of such vast changes in their position and
extension, as to have united them within the recent period to each
other and to the several intervening oceanic islands. I freely admit
the former existence of many islands, now buried beneath the sea, which
may have served as halting places for plants and for many animals
during their migration. In the coral-producing oceans such sunken
islands are now marked, as I believe, by rings of coral or atolls
standing over them. Whenever it is fully admitted, as I believe it will
some day be, that each species has proceeded from a single birthplace,
and when in the course of time we know something definite about the
means of distribution, we shall be enabled to speculate with security
on the former extension of the land. But I do not believe that it will
ever be proved that within the recent period continents which are now
quite separate, have been continuously, or almost continuously, united
with each other, and with the many existing oceanic islands. Several
facts in distribution, such as the great difference in the marine
faunas on the opposite sides of almost every continent, the close
relation of the tertiary inhabitants of several lands and even seas to
their present inhabitants, a certain degree of relation (as we shall
hereafter see) between the distribution of mammals and the depth of the
sea, these and other such facts seem to me opposed to the admission of
such prodigious geographical revolutions within the recent period, as
are necessitated on the view advanced by Forbes and admitted by his
many followers. The nature and relative proportions of the inhabitants
of oceanic islands likewise seem to me opposed to the belief of their
former continuity with continents. Nor does their almost universally
volcanic composition favour the admission that they are the wrecks of
sunken continents; if they had originally existed as mountain-ranges on
the land, some at least of the islands would have been formed, like
other mountain-summits, of granite, metamorphic schists, old
fossiliferous or other such rocks, instead of consisting of mere piles
of volcanic matter.
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