To summarize the message of this chapter, genes are selected, not for their intrinsic qualities, but by virtue of their interactions with their environments. An especially important component of a gene's environment is other genes. The general reason why this is such an important component is that other genes also change, as generations go by in evolution. This has two main kinds of consequences.
First, it has meant that those genes are favoured that have the property of 'cooperating' with those other genes that they are likely to meet in circumstances that favour cooperation. This is especially, though not exclusively, true of genes within the same species, because genes within one species frequently share cells with one another. It has led to the evolution of large gangs of cooperating genes, and ultimately to the evolution of bodies themselves, as the products of their cooperative enterprise. An individual body is a large vehicle or 'survival machine' built by a gene cooperative, for the preservation of copies of each member of that cooperative. They cooperate because they all stand to gain from the same outcome - the survival and reproduction of the communal body - and because they constitute an important part of the environment in which natural selection works on each other.
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.
This has been a difficult chapter, but it had to go into the book. Without it, we would have been left with the feeling that natural selection is only a destructive process, or at best a process of weeding-out. We have seen two ways in which natural selection can be a constructive force. One way concerns cooperative relationships between genes within species. Our fundamental assumption must be that genes are 'selfish' entities, working for their own propagation in the gene pool of the species. But because the environment of a gene consists, to such a salient degree, of other genes also being selected in the same gene pool, genes will be favoured if they are good at cooperating with other genes in the same gene pool. This is why large bodies of cells, working coherently towards the same cooperative ends, have evolved. This is why bodies exist, rather than separate replicators still battling it out in the primordial soup.
Bodies evolve integrated and coherent purposefulness because genes are selected in the environment provided by other genes within the same species. But because genes are also selected in the environment provided by other genes in different species, arms races develop. And arms races constitute the other great force propelling evolution in directions that we recognize as 'progressive', complex 'design'. Arms races have an inherently unstable 'runaway' feel to them. They career off into the future in a way that is, in one sense, pointless and futile, in another sense progressive and endlessly fascinating to us, the observers.
