We have seen that a very important part of a gene's environment is the other genes that it is likely to meet in successive bodies as the generations go by. These are the genes that are permuted and combined within the species. Indeed, a sexually reproducing species can be thought of as a device that permutes a discrete set of mutually accustomed genes in different combinations. Species, according to this view, are continually shuffling collections of genes that meet each other within the species, but never meet genes of other species. But there is a sense in which the genes of different species, even if they don't meet at close quarters inside cells, nevertheless constitute an important part of each others' environment. The relationship is often hostile rather than cooperative, but this can be treated as just a reversal of sign. This is where we come to the second major theme of this chapter, 'arms races'. There are arms races between predators and prey, parasites and hosts, even - though the point is a more subtle one and I shan't discuss it further- between males and females within one species .
