One of the most important and interesting retrospective emotions is revenge. The cardinal idea in revenge is returning evil for evil. Not only must there be a paying back for past injury, but there must be an equivalence, eye for an eye, and tooth for a tooth; and the revengeful emotion is the meting out such purely retributive action. Exact return becomes the basis of a general usage in animal and human societies. Justice, law, and punishment rest upon the idea of inflicting duplicate or equivalent injury for injury received. Administrative justice is the specialization of revenge in the hands of a few members of a community, a social differentiation by which individuals in general secure their revenges at great economy by proxy. Further, the revengeful emotion is a smouldering hate which vents itself only some time after the immediate occasion. This is not the flush of anger which prompts to vigorous offensive action upon the injurer at the very moment of harm perceived, and it does not appear as stimulant to immediate self-conservative activities, but is simply the spirit of getting even for relatively long past injury. What, now, is the function of revenge as a life factor? It surely does not mend my injury that I do another harm solely because he has some time harmed me, and the whole impulse might seem a pure waste of energy. But under natural selection revenge must arise in serviceability of some sort; and it is obvious that while revenge is of no use in mending the past, it yet has a large value with reference to future possible injury. Yet revenge is undeniably without conscious meaning for present or future; it is merely the spirit and determination to get even, and so its deterrent function is unconsciously attained. A dwelling in thought on the past _per se_, a feeling about it and acting on it, while it cannot help life directly, has a large value in its ultimate effect upon enemies. He who never forgets injury, and for whom by-gones are never by-gones, who never fails to return injury for injury, is feared and is less likely to be injured. Junker, the African traveller, remarks of the pygmies, “They are much feared for their revengeful spirit.” Thus, other things being equal, the most revengeful are the most successful in the struggle for self-conservation and self-furtherance. Though by itself considered irrational and foolish to inflict return injuries upon an injurer long after the immediate occasion, yet its deterrent effect is very great with reference to other assailants. Thus, pure retrospection may have unconsciously prospective value, or sometimes revenge may be really retrospective-prospective, as when one says, “I will fix him so he will not do that again.” Here function is consciously known, but in instinctive revenge there is no such foresight, and, in general, utility is no consideration with the revenger, whose mind is bent rather on doing great harm for its own sake to his enemy rather than benefiting himself.