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 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 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.