3. _Each species of the lower animals is seen to have a relatively fixed habitat limited by its food-supply and by its enemies._ The rocks tell a 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 convulsions of nature or to interference by man with the natural conditions. But the usual condition is an equilibrium of numbers, long maintained, though each species appears to have in itself a capacity for unlimited increase. Why this contradiction? The limit set by the food-supply is seen in a simple case when herbivorous animals are placed on an island from which they cannot escape, and where there are no dogs, wolves, weasels, or foxes. Substantially this experiment was unintentionally tried on an enormous scale with the rabbit in Australia. This peculiar and long-isolated continent contained none of the rabbit's ancient enemies. The rabbits became a pest, devastated great areas, were hunted, trapped, poisoned, and great numbers of them died of starvation outside the fences erected to stop their advance. In the imaginary island they would increase up to the point where starvation would bring about an equilibrium between the number of animals and the food supply. The destruction of one kind of animal by another limits numbers in another way. The number 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 on one side, it decreases on the other, until an equilibrium is reached. [Sidenote: The surplus of life germs] Throughout nature each species of animal keeps its customary place, changing little despite its efforts to increase and to crowd into the habitat of other species. 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 full age. The myriads of frogs born every spring, the swarms of insects, the countless plants, are struggling to find a foothold on the crowded earth. Of the vastly greater number of seeds and embryos, only one in a multitude ever comes or could come to maturity. Here are the undisputed facts on which rests a biologic "doctrine of population," so to speak, for the vegetable and lower animal world. Because of the limited powers of the soil, no form of life, animal or vegetable, can continue to increase even for a single generation, without meeting enormous forces of opposition, which destroy great numbers and set a limit to the increase of the species. [Sidenote: These facts related to the doctrine of population]