Because natural selection evaluates currently available variation against current conditions, Darwinian evolution is a process of the moment; it does not see beyond today. Its response to a new demand or new opportunity is to work with what it has, fitting like a good backyard mechanic whatever suitable components are at hand into elegant new adaptations. Because its resources are limited, it sometimes fails and extinction follows, but conditions rarely change dramatically and organisms are flexible; what is here today much resembles what was also here yesterday.

The term environment might be misunderstood as excluding the biological interactions that operate within it, competition for space, food and shelter between individuals for example, or the impact of predators. The biological world is intricately woven together with the non-living environment, and plays as large a role as the motor or the acceptor of change. This is a book about oceans and continents, however, about the face of the earth as it has changed over the eons. When we address here the interaction of the changing environment with the history of life, we are mainly concerned with the impact of the non-living environment on the living one and vice versa.

On a grand scale, the opportunities presented by the earth to its occupants are, or rather were, three. First, there was once, but is no more, the potential of vast, empty territories.