ABSTRACT
In the preceding four chapters I have examined the genetic basis of evolution. I have tried to show that the key to the entire process rests in two sets of opposing mechanisms. One of these sets operates to maintain stability; the other provides the variation necessary for further adaptation. Evolution is an adjustive process in which species respond to continuous environmental pressures. In one sense the response is passive, since it depends upon existing random variation; but in another sense it is directional, since adaptive trends (or patterns of fit) develop in relation to particular environmental settings.
