In the same manner that the individual is liable to changes through the action of external agencies, and offers no resistance thereto, nor any indication of the possession of a physiological inertia, but submits at once to any impression, so likewise it is with aggregates of men constituting nations. A national type pursues its way physically and intellectually through changes and developments answering to those of the individual, and being represented by Infancy, Childhood, Youth, Manhood, Old Age, and Death respectively. [Sidenote: Disturbance through emigration.] But this orderly process may be disturbed exteriorly or interiorly. If from its original seats a whole nation were transposed to some new abode, in which the climate, the seasons, the aspect of nature were altogether different, it would appear spontaneously in all its parts to commence a movement to come into harmony with the new conditions--a movement of a secular nature, and implying the consumption of many generations for its accomplishment. During such a period of transmutation there would, of course, be an increased waste of life, a risk, indeed, of total disappearance or national death; but the change once completed, the requisite correspondence once attained, things would go forward again in an orderly manner on the basis of the new modification that had been assumed. When the change to be accomplished is very profound, involving extensive 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.