Each new genetic improvement selected on one side of the arms race – say predators – changes the environment for selection of genes on the other side of the arms race – prey. It is arms races of this kind that have been mainly responsible for the apparently progressive quality of evolution, for the evolution of ever-improved running speed, flying skill, acuity of eyesight, keenness of hearing, and so on. These arms races don’t go on forever, but stabilize when, for instance, further improvements become too economically costly to the individual animals concerned.
