The origin and progress of the idea that murder is a crime can almost be traced step by step. The wife of a rude savage does something which offends him; a violent perception of anger flashes from the visual organ to the perceptive area of the brain, and a reflex action flashes from it along the motor nerve to the muscles of the arm. He strikes and kills her, almost as unconsciously and instinctively as he walks or breathes. But other perceptions follow on the act. He finds next day that he has no one to cook his food; the image of her dying face photographed on his brain is an unpleasant one; and thus by degrees a series of secondary perceptions get attached to the primary one of striking when he feels angry. If he gets another wife who again provokes him, the primary perception calls up the secondary ones, and the nerve-centres of his brain, instead of being solicited only in one direction, are acted on in opposite ways by conflicting impressions. He hesitates, and, as the primary impulse of passion is probably the more evanescent, the restraining impulses prevail, and every time they prevail they acquire more strength. Gradually they extend to a conviction that it is both inconvenient and disagreeable to kill any one with whom he is closely related either by family or tribal ties, and that, in a word, murder does not pay, and is wrong, unless practised on an enemy. This idea accumulates by heredity, and evidently those tribes or races in whom it is strongest will have an advantage in the struggle for life and be most likely to survive. From this point the idea may be traced historically, deepening and widening from generation to generation as civilisation advances, until in the higher races it assumes the form of an instinctive abhorrence of murder in the abstract, as we find it at the present day.