I shall, however, presently have
to discuss this branch of the subject in some detail. Changes of level in
the land must also have been highly influential: a narrow isthmus now
separates two marine faunas; submerge it, or let it formerly have been
submerged, and the two faunas will now blend or may formerly have blended:
where the sea now extends, land may at a former period have connected
islands or {357} possibly even continents together, and thus have allowed
terrestrial productions to pass from one to the other. No geologist will
dispute that great mutations of level have occurred within the period of
existing organisms. Edward Forbes insisted that all the islands in the
Atlantic must recently have been connected with Europe or Africa, and
Europe likewise with America. Other authors have thus hypothetically
bridged over every ocean, and have united almost every island to 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 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
{358} 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 fact