In this struggle for existence, what plants and animals survive? The answer is that those which are best fitted to their environment continue to exist and to reproduce themselves, while those which are more poorly adapted to their environment fall out of the race and disappear. [Sidenote: The survival of the fittest.] In other words natural selection or the survival of the fittest was thought by Darwin to be the principle which determines the course of evolution. The unfit perish and the fit survive, everything depending upon the relative success of the organism in adapting itself to the conditions under which it is endeavoring to live. The clumsy mastodon became extinct; his bones are now relics in museums; but the horse, being fleet of foot, managed to survive. The fit organisms,—plants or animals or human beings,—have survived and have perpetuated the species. They gave to their offspring the traits or qualities which enabled themselves to survive. In that way each generation of organisms became a little better fitted to its environment than the generation which went before. This is a slow process for human beings, of course, for it takes twenty years or more to produce a new generation of men, whereas new generations of birds, reptiles, and lower animals appear every few months. The principle of natural selection, moreover, does not fully account for the form which evolution has taken. Other factors have also been at work, but scientists are not yet agreed as to their nature or importance.[4] [Sidenote: Natural selection as applied to the human race.] Now how does the human race figure in all this? Mankind has also been at all times under the necessity of adapting itself to its environment, and in the early stages of human history those who did not successfully adapt themselves went to an early grave. During century after century natural selection and the other factors strengthened the race. As the race grew stronger in intelligence, man undertook to subdue his environment rather than to be subdued by it, and in considerable measure he succeeded. He discovered the art of kindling a fire and made this element his servant in conquering the cold. He domesticated wild animals, made them provide him with milk and meat, and compelled them to carry his burdens. Step by step he mastered the natural conditions which surrounded him. This he did by his ability to work with his fellow-men. Through this power of co-operation he created group organizations—society, the state, and government. [Sidenote: Today the strong assist the weak.]