Second, circumstances don't always favour cooperation. In their march down geological time, genes also encounter one another in circumstances that favour antagonism. This is especially, though not exclusively, true of genes in different species. The point about different species is that their genes don't mix - because members of different species can't mate with one another. When selected genes in one species provide the environment in which genes in another species are selected, the result is often an evolutionary arms race. 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.
