4. _A doctrine of human population is a reasoned explanation of the causes determining the number of people in the world._ Man in his economic life is constantly struggling with the problem of the scarcity of goods. If in any given environment men continue long to increase, they must, like the lower animals, meet limits in the capacities of the resources they use. The supply of labor force which is thus brought to be combined with the material agents must meet with diminishing returns unless these agents also continue to increase at a like rate. The relation of population to resources thus presents probably the most fundamental problem in the realm of economics. It is a problem of great complexity, bristling with difficulties, and incapable of exact mathematical treatment; but it is capable of rational study. There is a great difference between a purely fatalistic view of this question and the view that is to be reached by a consideration of the motives, causes, and physical influences at work; It is possible to find some principles in the chaos of prejudices and contradictions that the subject presents. The fruit of a century of discussion of the economic, social, and biologic factors involved, is a rational, if not a final, doctrine of population. § II. POPULATION IN HUMAN SOCIETY [Sidenote: The biologic stage of human population] 1. _In the earlier stages of human history, population is limited mainly by biologic factors._ The biologic stage continues so long as there are no artificial restraints put on the birth-rate, and no deliberate destruction of offspring for the purposes of limiting the size of the family. There the limits are all objective; they are found in scantiness of the food-supply, or in destruction by enemies, animal or human. Each species has an average or normal birth-rate, great or small. Just why this varies, why the rabbit produces a score of young in a year, and the elephant but one in three years, is a question capable of a rational answer, but it is one for the natural scientist rather than for the economist. Each species is impelled by instinct to realize this birth-rate, to bring into existence as many young as possible. No human society known to us is so primitive that it has not passed this stage, but many societies have risen but little above it. In most savage tribes, where starvation, disease, and war are constantly at work, the difficult task is to maintain the population. Few of those born arrive at maturity. The custom of the adoption of captives from hostile tribes is widespread, because the efficiency and even the survival of the tribe depends upon keeping up its number of warriors. [Sidenote: War among primitive societies]