Chap, XI. 
MEANS OF DISPEKSAL, 
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 wuthin 
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 inter¬ 
vening oceanic islands. I freely admit the former ex¬ 
istence 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 distri¬ 
bution, we shall be enabled to speculate with security 
on the former extension of the land. But I do not 
believe that it wiU ever be proved that within the 
