(1) They are in part due to certain organic needs and cravings which exist independently of the individual's environment. Hunger and thirst imperiously check the tendency to laziness, or heedlessness, and stimulate to industriousness and prudence. To this day the mere need of food and clothing and shelter is the main bulwark of these virtues. The acquisitive impulse, which is also rather early in appearance, has an increasing share in this sort of moralization. The craving for action, which is the natural result of abundant nervous and muscular energy, the combative instinct, the joy of conquest and achievement, and the sexual impulse, go far in counteracting cowardice and inertia. The artistic impulse, when it emerges in man, long before the dawn of history, makes against caprice for orderliness, self-control, and patience. Ambition is a potent force in human affairs. The desire for the approval of others, which is prehuman, makes for all the virtues. (2) But in addition to these inward springs of morality there is the constant pressure of a hostile environment. Cold, storms, rivers that block journeys, forests that must be felled, treacherous seas that lure with promise and exact toll for carelessness, arouse men out of their torpor and aid the development of the virtues we have been considering. The necessity of rearing some sort of shelter makes against laziness for industry and perseverance. The dangers of wind or flood check heedlessness in the choice of location for the home and foster prudence and foresight. In the harsher climates man is more goaded by nature; hence more moral progress has, probably, been effected in the temperate than in the tropical zones. (3) A third and very important source lies in the mutual hostility of the animal species and of men. Slothfulness and recklessness mean for the great majority of animals the imminent risk of becoming the prey of some stronger animal. Among tribes of men the ceaseless struggles for supremacy have pricked cowardice into courage, demanded self-control instead of temper, supplanted gluttony and drunkenness by temperance. Cruel as has been the suffering caused by war, and deplorable as most of its effects, it did a great deal in the early stages of man's history to promote the personal virtues, alertness, moderation, caution, courage, and efficiency.