Change of
climate must have had a powerful influence on migration. A region now
impassable to certain organisms from the nature of its climate, might have
been a high road for migration, when the climate was different. 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 together, 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 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.
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