Man, being an animal, is under the influence of all the natural laws governing the evolution, increase and spread of other species. The possession of intelligence is generally, though falsely, assumed to upset law, yet the brain is a material thing, after all is said, and its functions are natural phenomena. Intelligence merely makes new phenomena and modifies old ones, but does not change any laws. The invention of balloons and aeroplanes has not upset gravitation. The great saurians once overran the earth, but later a few frail mammals became dominant because possessed of enough intelligence to survive conditions which killed the more stupid saurians. In like manner man became the dominant mammal, and the more intelligent races have long been exterminating or controlling the lower.
The basic fact, governing the evolution and spread of any species, is the fact that many more individuals are born than can possibly survive to raise offspring of their own. The surplus are killed off in one way or another, and only the fittest for survival remain. If these fittest are markedly different from the parent forms there is a new species, though biologists have not yet determined why offspring vary from ancestral types.
It is amazing that though man's enormous birth rate has been under discussion for over a century - ever since the epoch-making essay on Populations by Thomas R. Malthus - yet the full significance of the phenomenon has never been realized. Darwin himself called attention to the fact that unless we are thinned out by war, famine, disease and other accidents, our birth rate would so populate the earth in a few centuries that there would not be standing room. That is, in accordance with the laws governing all living forms, our birth rate is so large that there is chronic overpopulation, and always has been. The fact has been ignored and even denied, probably because of our neglect to apply to man to their fullest extent, all the laws governing his existence. Until we coldly consider him as merely one of the earth's animals, it is not possible to understand the past and present expansion of races.
Like every other organism, man spreads over the earth in search of food, for if he can obtain subsistence he is loth to move. Consequently there can be but one reason for shifting of residence - relative overpopulation at the native place.
It is a biological axiom that every part of the earth supports as many living things as can find food, or that the earth is always saturated with life. This does not mean that all the food is consumed, for that would result in extermination of supplies - some must be left for reproduction, and some may be unattainable.
The number of individuals in any species does not vary much from year to year, for if there are too many, some must starve, and if there are too few, they have increased chances of rearing offspring, and the balance is restored at once. If the numbers are too great they also destroy their own food supply, and in subsequent years the saturation point is much reduced. A visitation of a swarm of locusts, for instance, may so far destroy vegetation, that no locusts can exist in that place for months or years. Again, an increase in numbers of one species, is soon followed by an increase of the enemies which prey upon it, for every species of living thing is a food for some other. The increase of enemies destroys this increase, then the enemies die out from lack of food, and both species revert to their normal saturation points or the maximum numbers which can obtain food. Averaging up all this constant shifting and balancing of food and feeders, we see that in the long run the death rate in any species is exactly equal to the birth rate - a law which holds universally for surviving species, and which cannot be persistently violated without one of two results - either the species will eventually overrun the earth, crowding out all competitors, or else it dies out. The law holds irrespective of the birth rate, and is as true for elephants, which have but few offspring, as for the fish, which have thousands, for in each case the number of deaths is equal to the births. After the two old elephants die they are succeeded by but two survivors of all their children, and similarly the myriads of young fish all die but two.