It is impossible for human beings fully to understand either themselves
or their long prehuman history without knowing something of the
process (or, rather, processes) by which our remarkable species became
what it is. This is, as (almost) everybody knows, evolution. And
although most of us have a vague idea of what evolution is all about, few
realize quite how many factors have typically been involved in the evolutionary
histories that gave rise to the diversity of today’s living world.
For evolution is not, as we often believe, a simple, linear process; rather,
it is an untidy affair involving many different causes and influences