"Dawkins’s carefully worded passage also explains why nature does not consist of one big bloody melee. For one thing, animals are less inclined to harm their close relatives, because any gene that would nudge an animal to harm a relative would have a good chance of harming a copy of itself sitting inside that relative, and natural selection would tend to weed it out. More important, Dawkins points out that another organism differs from a rock or a river because it is inclined to hit back. Any organism that has evolved to be violent is a member of a species whose other members, on average, have evolved to be just as violent. If you attack one of your own kind, your adversary may be as strong and pugnacious as you are, and armed with the same weapons and defenses. The likelihood that, in attacking a member of your own species, you will get hurt is a powerful selection pressure that disfavors indiscriminate pouncing or lashing out. It also rules out the hydraulic metaphor and most folk theories of violence, such as a thirst for blood, a death wish, a killer instinct, and other destructive itches, urges, and impulses. When a tendency toward violence evolves, it is always strategic. Organisms are selected to deploy violence only in circumstances where the expected benefits outweigh the expected costs. That discernment is especially true of intelligent species, whose large brains make them sensitive to the expected benefits and costs in a particular situation, rather than just to the odds averaged over evolutionary time."