=The multiplication and crowding of animals.=--In the reproduction or multiplication of animals the production of young proceeds in geometric ratio, that is, it is truly a multiplication. Any species of animal, if its multiplication proceeded unchecked, would sooner or later be sufficiently numerous to populate exclusively the whole world. The elephant is reckoned the slowest breeder of all known animals. It begins breeding when thirty years old and goes on breeding until ninety years old, bringing forth six young in the interval, and surviving until a hundred years old. Thus after about eight hundred years there would be, if all the individuals lived to their normal age limit, 19,000,000 elephants alive descended from the first pair. A few years more of unchecked multiplication of the elephant and every foot of land on the earth would be covered by them. But the rate of multiplication of other animals varies from a little to very much greater than that of the elephant. It has been shown that at the normal rate in increase in English sparrows, if none were to die save of old age, it would take but twenty years to give one sparrow to every square inch in the State of Indiana. The rate of increase of an animal, each pair producing ten pairs annually and each animal living ten years, is shown in the following table: Years. Pairs produced. Pairs alive at end of year. 1 10 11 2 110 121 3 1,210 1,331 4 13,310 14,641 5 146,410 161,051 10 ...... 25,937,424,600 20 ...... 700,000,000,000,000,000,000 Some animals produce vast numbers of eggs or young; for example, the herring, 20,000; a certain eel, several millions; and the oyster from 500,000 to 16,000,000. Supposing we start with one oyster and let it produce one million of eggs. Let each egg produce an oyster which in turn produces[19] one million of eggs, and let these go on increasing at the same rate. In the second generation there would be one million million of oysters, and in the fourth, i.e. the great great grandchildren of the first oyster, there would be one million million million million of oysters. The shells of these oysters would just about make a mass the size of the earth.