There are important parallels between the pattern of inference Kenneth Waltz uses in his Theory of International Politics and early Darwinian reasoning. This early Darwinian thinking has needed to be significantly refined by modern evolutionary biologists, and their amendments are equally relevant to Waltz’s model. Waltz allows for states to imitate each other, and also accepts that they are only rarely eliminated from the system. Modern Darwinian analyses show that where elimination is rare and imitation common it is quite possible for deleterious behaviours to become widespread. We cannot assume an anarchic system will select for security-enhancing behaviours in major powers. Thinking about Waltz’s argument in these terms opens space to disagree with his conclusions while respecting the strength of his logic.