In compliance with the custom which obliges one to apologize for presenting to the public anything new, it must be explained that this work is an anthropological study of one of the reasons for migration, war, famine and pestilence, and why mankind, in obedience to natural law, is unconsciously organizing to prevent these disasters and to make it possible for every babe to reach old age - excepting those meeting unavoidable fatal accidents, and even these become avoidable as knowledge increases.<br><br>Harmful customs, cannot persist or they would destroy the species. War has survived because its advantages were greater than its disadvantages, and it is an instance of the survival of the fittest custom. There is no doubt, nevertheless, that on account of its disadvantages it is being constantly replaced by other methods cheaper in life and money, but which serve the same ends - survival of the most - and in time national wars will cease, but it will be a long time, for such a consummation requires a world-wide organization. In the absence of war there are other factors which prevent survival of all children, and necessitate a large birth rate.<br><br>The work, therefore, takes up the reasons for the increase, spread and organization of populations, together with the checks to overpopulation. It merely applies to man the natural laws which are known to govern the spread of all other species of plant or animal. It then explains the relationships of higher and lower races of man, and shows why we expanded across the Atlantic to America in the sixteenth and seventeenth centuries, and thence across the Pacific, and why the higher races must always control the tropics, though acclimatization and colonization are not possible.