The mirror has indeed two faces.
As with all gregarious animals, 'two souls,' as Faust says, 'dwell with-in his breast,' the one of sociability and helpfulness, the other of jealousy and antagonism to his mates.
Though in a general way he cannot live without them, yet, as regards certain individuals, it often falls out that he cannot live with them either.
Constrained to be a member of a tribe, he still has a right to decide, as far as in him lies, of which other members the tribe shall consist. Killing off a few obnoxious ones may often better the chances of those that remain. And killing off a neighboring tribe from whom no good thing comes, but only competition, may materially better the lot of the whole tribe.
Hence the gory cradle, the bellum omnium contra omnes , in which our race was reared; hence the fickleness of human ties, the ease with which the foe of yesterday becomes the ally of to-day, the friend of to-day the enemy of to-morrow.
hence the fact that we, the lineal representatives of the successful enactors of one scene of slaughter after another, must, whatever more pacific virtues we may also possess, still carry about with us, ready at any moment to burst into flame, the smoldering and sinister traits of character by means of which they lived through so many massacres, harming others, but themselves unharmed.