§ 10. Inadequate recognition of psychic factors. Indeed the work of Malthus is replete with suggestions and with keen observations of history, not sufficiently analyzed or or ganized, and often expressed in ambiguous terms. The Mal thusian doctrine of population has been the center of a con tinuous, and often bitter, controversy ever since its appear ance, both as matter of economic theory and as to its bearing upon radical social reforms. Because of the features in it we have just noted it is futile now to line up for or against Malthusianism. Hardly any two persons mean just the same thing by that term. We may agree that Malthus got hold of a great biological principle without understanding its full bearings. Darwin was struck by this in reading Malthus' book, and made it the starting point of his great doctrine of natural selection in explaining evolution. Since 1859, there fore, we are in a position to see the subject in much broader perspective. It is best therefore to put aside from our thoughts prejudices and controversies clustering around the Malthusian doctrine, and to find a basis for a doctrine of pop ulation in the ideas of modern biology and psychology and in the statistical facts of our times.
§ 11. The overplus of blossom. A doctrine of population is the grouping and explanation of the various influences that combine to determine the number of people in the several localities and in the world as a whole. The most fundamental fact in the doctrine of population is the surplus of life germs. In every species of living organism, vegetable or animal, the production of germ cells in each generation is vastly greater than the number that develops into living offspring. Yet the number of offspring born is much greater, in most species vastly greater, than would suffice to maintain the number of living individuals undiminished if all the young lived to ma turity. Each species has an average or normal birth rate, great or small. Insects produce thousands of eggs each, fish produce hundreds, the rabbit a score of young in a year, and the elephant but one in three years. Clearly there is a gen eral inverse relation between the intelligent care that the parents of any species give to their offspring, and the number of life germs produced. Nature economizes the forces of the species by a gradual reduction of the real surplus, but always leaves what the engineers call a "factor of safety." There are so many chances of accident, that if the number of germs is enough only in favorable conditions, the species will become extinct under any conditions in the least unfavorable.
seeking for a chance to germinate, these myriads of young in every species seeking to survive, can not possibly grow to maturity. Even the slow-breeding elephant, with a period of gestation of three years, and producing one calf at a birth, would cover the entire earth and leave no standing room in a few centuries if every calf born could live to maturity. In how much briefer time would the fast-breeding animals and insects cover every foot of the earth ! The limit of the food supply alone would prevent this. This has been demonstrated repeatedly when herbivorous animals have been placed on an island from which they could not escape, and where there were no large beasts of prey. This demonstration was made on an enormous scale when the rabbit was introduced into Aus tralia, that peculiar and long isolated continent containing none of the rabbit's ancient enemies. The rabbits increased and devastated great areas, and tho they have been hunted, trapped, and poisoned by the millions, and great numbers of them have died of starvation outside the wire fences erected to stop their progress, they still continue to be a pest. If there were no other limit earlier interposed, this ultimate limit of the food supply would quickly check the increase of any form of animal life.
§ 13. Eater and eaten. The destruction of one kind of animal by another limits numbers in another way. The num ber of lions is limited by the number of their prey in the region where they roam. The number of deer, therefore, is limited in two ways, by the amount of their food and by the number of lions which catch the deer. The more numerous the lions, the fewer the deer; the fewer the deer, the greater the supply of vegetable food ; as the pressure increases one side, it decreases on the other, until an equilibrium is reached. Some carnivorous animals will in times of great hunger eat the weaker members of their own species, even their own young. The actual limitation here is thus not starvation, but violence induced by hunger.
Geology tells the story of a slow and steady change that has gone on in the earth and in the species of animals that inhabit it. History records some rapid changes due to con vulsions of nature or to interference by man with the natural conditions. But the usual condition is an equilibrium of numbers, long maintained. Tho each species of animal has a capacity for unlimited multiplication, throughout nature each keeps its customary place, changing little despite its ef forts to increase and to crowd into the habitat of other species.