st disputes that great mutations
of level have occurred within the period of existing organisms. Edward
Forbes insisted that all the islands in the Atlantic must have been
recently connected with Europe or Africa, and Europe likewise with
America. Other authors have thus hypothetically bridged over every
ocean, and united almost every island with some mainland. If, indeed,
the arguments used by Forbes 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 in the
level of the land or sea; but not of such vast changes in the position
and extension of our continents, 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 by rings of coral or atolls standing
over them. Whenever it is fully admitted, as 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 most of our continents which now
stand 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--the degree of affinity between the mammals
inhabiting islands with those of the nearest continent, being in part
determined (as we shall hereafter see) by the depth of the intervening
ocean--these and other such facts are opposed to the admission of such
prodigious geographical revolutions within the recent