Confined fluids which do not move as a whole, are still in internal motion, and there is a restless "convection" by which internal currents cause a ceaseless mixing of particles. In the same way there are minor currents in each nation at peace, or from nation to nation, causing a ceaseless movement of mixing, so that the inhabitants of a district are constantly changing. The longer a confined fluid is kept at rest, the less become the currents of convection, and the longer a people is at peace the quieter it becomes, and the more steadfast is the population of any one district. Our own spreading currents were comparatively feeble and sluggish until stirred up by the Mexican War and the discovery of gold in the West, when they became tremendous. Currents which overflow political boundary lines are now just as constant as ever, but they are more evidently overflows. It seems as though the effect of modern governments is to build high walls along the borders to confine the fluid so that it is deeper, like a mill pond. Thus, population may be dense in one place and thin in another, but no marked mixing occurs as of old, until the fluid rises above the retaining walls and the surplus flows over. If the counter pressure on the other side should diminish, the wall might break and the fluid pour out, like the recent German flood into Alsace-Lorraine. In that case the nations build new retaining walls around the flooded territory.
A homogenous population, if allowed to rest, settles itself into layers like liquids of different densities, and this phenomenon is due to the normal variations in brain power. There is an upper, middle and lower class in every nation, except possibly among the lowest savages. We have our submerged tenth, and our best people, just as they have in England; and likewise in both countries, there are constant currents carrying men from one class into another. After a few generations in an upper or a lower class, the descendants are apt to die out or seek the great middle class - the real people. It is usually three generations from shirt sleeves to shirt sleeves, in spite of some notable exceptions.
We may pour unmisciblc fluids into a bottle and shake them together to get an emulsion of separate particles, but the fluids will settle into layers again. Similarly immiscible populations forcibly shaken together will naturally separate. This is best illustrated in the Pacific Islands, where the original black, short, woolly-haired negritto type in the North, and the Melanesians, a similar type in Australasia, were the original inhabitants. They were forced to the hills or subdued by the second or pre-Malay race. A third people, the Malays, leaving Asia comparatively recently, have forced themselves in here and there. These three types have been enormously mixed, and yet are as distinct as they were hundreds of years ago. Japanese and Chinese have spread to the Pacific Islands and mixed with the rest of the population, and yet the frail half-breed types gradually disappear. It seems as though the various races of man were almost of different species, for by the law so mush used by Darwin, different varieties of the same species, such as the domestic dog, produce vigorous fecund offspring, while hybrids (like mules) between animals of different species, are sterile or soon die out. Though the Negro and the Aryan had the same ancestor, if we go far enough back geologically, they have diverged so much as to be almost different species instead of being different varieties of the same as usually taught - indeed, a few scientists believe them to be different species.