going on, there is no difference of opinion. Every successive
stratum of sedimentary rock, sand, or gravel, is a proof that
changes of level have taken place; and the different species of
animals and plants, whose remains are found in these deposits,
prove that corresponding changes did occur in the organic world.

Taking, therefore, these two series of changes for granted, most
of the present peculiarities and anomalies in the distribution of
species may be directly traced to them. In our own islands, with
a very few trifling exceptions, every quadruped, bird, reptile,
insect, and plant, is found also on the adjacent continent. In
the small islands of Sardinia and Corsica, there are some
quadrupeds and insects, and many plants, quite peculiar. In
Ceylon, more closely connected to India than Britain is to
Europe, many animals and plants are different from those found in
India, and peculiar to the island. In the Galapagos Islands,
almost every indigenous living thing is peculiar to them, though
closely resembling other kinds found in the nearest parts of the
American continent.

Most naturalists now admit that these facts can only be explained
by the greater or less lapse of time since the islands were
upraised from beneath the ocean, or were separated from the
nearest land; and this will be generally (though not always)
indicated by the depth of the intervening sea. The enormous
thickness of many marine deposits through wide areas shows that
subsidence has often continued (with intermitting periods of
repose) during epochs of immense duration. The depth of sea
produced by such subsidence will therefore generally be a measure
of time; and in like manner, the change which organic forms have