lly unjustifiable assumption that no dry land rose elsewhere when
our present dry land sank, there must be half-a-dozen Atlantises
beneath the waves of the various oceans of the world. But if the
regions which have undergone these slow and gradual, but immense
alterations, were wholly or in part inhabited before the changes I
have indicated began--and it is more probable that they were, than
that they were not--what a wonderfully efficient "Emigration Board"
must have been at work all over the world long before canoes, or even
rafts, were invented; and before men were impelled to wander by any
desire nobler or stronger than hunger. And as these rude and primitive
families were thrust, in the course of long series of generations,
from land to land, impelled by encroachments of sea or of marsh, or
by severity of summer heat or winter cold, to change their positions,
what opportunities must have been offered for the play of natural
selection, in preserving one family variation and destroying another!

Suppose, for example, that some families of a horde which had reached
a land charged with the seeds of yellow fever, varied in the direction
of woolliness of hair and darkness of skin. Then, if it be true that
these physical characters are accompanied by comparative or absolute
exemptions from that scourge, the inevitable tendency would be to the
preservation and multiplication of the darker and woollier families,
and the elimination of the whiter and smoother-haired. In fact, by the
operation of causes precisely similar to those which, in the famous
instance cited by Mr. Darwin, have given rise to a race of black pigs
in the forests of Louisiana, a negro stock would eventually people the
region.

Again, how often, by such physical changes, must a stock have been
isolated from all others for innumerable generations, and have found
ample time for the hereditary hardening of its special peculiarities
into the enduring characters of a persistent modification.

Nor, if it be true that the physiological difference of species may be
produced by variation and natural selection, as Mr. Darwin supposes,
would it be at all astonishing if, in some of these separated stocks,
the process of differentiation should have gone so far as to give
rise to the phenomena of hybridity. In the face of the overwhelming
evidence in favour of the unity of the origin of mankind afforded by
anatomical considerations, satisfactory proof of the existence