tensive anatomical alterations not merely
in the appearance of the skin, but even in the structure of the skull,
long periods of time are undoubtedly required, and many generations of
individuals are consumed.

[Sidenote: And through blood admixture.]

Or, by interior disturbance, particularly by blood admixture, with more
rapidity may a national type be affected, the result plainly depending
on the extent to which admixture has taken place. This is a disturbance
capable of mathematical computation. If the blood admixture be only of
limited amount, and transient in its application, its effect will
sensibly disappear in no very great period of time, though never,
perhaps, in absolute reality. This accords with the observation of
philosophical historians, who agree in the conclusion that a small tribe
intermingling with a larger one will only disturb it in a temporary
manner, and, after the course of a few years, the effect will cease to
be perceptible. Nevertheless, the influence must really continue much
longer than is outwardly apparent; and the result is the same as when,
in a liquid, a drop of some other kind is placed, and additional
quantities of the first liquid then successively added. Though it might
have been possible at first to detect the adulteration without trouble,
it becomes every moment less and less possible to do so, and before long
it cannot be done at all. But the drop is as much present at last as it
was at first: it is merely masked; its properties overpowered.

Considering in this manner the contamination of a numerous nation, a
trifling amount of foreign blood admixture would appear to be indelible,
and the disturbance, at any moment, capable of computation by the
ascertained degree of dilution that has taken place. But it must not be
forgotten that there is another agency at work, energetically tending to
bring about homogeneity: it is the influence of external physical
conditions. The intrusive adulterating element possesses in itself no
physiological inertia, but as quickly as may be is brought into
correspondence with the new circumstances to which it is exposed, herein
running in the same course as the element with which it had mingled had
itself antecedently gone over.

National homogeneity is thus obviously secured by the operation of two
distinct agencies: the first, gradual but inevitable dilution; the
second, motion to come into harmony with the external natural state. The
two consp