Some animals tend to multiply ten fold in a generation; some would multiply to a hundred fold, and others to ten thousand fold. Not only could the progeny of any land animal, if given the right of way, soon crowd the earth, but the fishes, under favorable conditions, would, in a short time, literally fill the sea. A million eggs are spawned by the cod-fish in order that two cod-fish may reach maturity. The eel also spawns millions of ova that a few eels may reach the spawning age. It is clear, therefore, that if all these eggs produced fishes, and if these reached maturity and in their turn multiplied with such lavish prodigality, a few generations would suffice to transform the ocean into a solid mass. The same phenomenon obtains in the plant world. Every plant gives its seeds to the soil as Autumn’s spendthrift hand gives its withered leaves to the wind, but of the millions of seeds thus scattered, comparatively few take root and mature and reproduce their kind. Notwithstanding Nature’s tremendous efforts to overwhelm the earth with the creatures of every species, the checks upon over-population are so numerous and so effective that, speaking generally, the number of animals and plants in any given area where the conditions are unchanging remains fairly constant. This relatively rigid limitation of the number of creatures that shall survive in any environment is due to the operation of Nature’s efficient machinery whose grim function is the destruction of life. Millions of creatures live only by devouring other creatures. The carnivorous animals eat the flesh and drink the blood of the herbivorous. The larger carnivores feed upon the smaller. That the badger may make a meal, a whole nest of bees must be destroyed. The ant-eater swallows a tongue load of ants at a single gulp. The eagle and the hawk dine on doves and other birds. To the appetite of smaller birds are sacrificed worms, insects and larvae. Big fishes eat the little ones; bigger fishes eat the eaters and in turn are eaten. On land, in the air, and in the sea the incalculable sacrifice of life proceeds without cessation. Pursuit and flight, capture and death—a tragedy on which the curtain never falls! This is the struggle for existence. In this struggle which, in a sense, is a constant test of skill, the most highly qualified creatures are the ones best adapted to catch their prey or to escape their enemies. Differing from their parents and from one another in numerous variations, some animals excel others in important respects—in having stronger legs with which to run, better eyes with which to see, a keener sense of smell, hearing more acute, and teeth and claws better fitted to hold and rend their prey. To the animal that would obtain food or avoid being eaten, some or all of these characters are prime essentials. In the struggle for existence the animals possessing these attributes in superior form will survive while those in which they are less developed must perish.