There are words which some turn in the history of thought suddenly overwhelms with a large special meaning. Such a one is evolution. We might prefer to qualify it as ‘biological evolution’, were it not that, scientifically speaking, the distinction between ‘living’ and ‘lifeless’ seems somewhat strained. The special connotation attaching to evolution implies that types of living things have, like human affairs, a history which can be traced, and that when traced their history bears witness to a progressive change whereby what they are is different from what they were. New types, the expression runs, have been’ evolved’. In the course of their history, in many cases more complex types have evolved from less complex. And that aspect of evolution is of particular interest to ourselves because we ourselves seem to be among its instances. We seem an outstanding example of it.