Ernst Mayr was one of the leading evolutionary biologists of the last century. He lived to be a hundred years old and was well into his nineties when he wrote this book, showing that his brain was as sharp as ever.
It is a wide-ranging book which succinctly covers all the key issues relating to evolution, but I would particularly like to draw attention to two aspects of Mayr's approach.
Firstly, he shows that Darwinian evolutionary theory actually consists of several distinct strands. For example, after the publication of Darwin's "On the Origin of Species", the idea of the common descent of all living creatures ("the tree of life") became widely accepted relatively quickly, whereas the mechanism of evolutionary change - natural selection - took much longer to gain widespread support, even amongst scientists.
Similarly, Mayr shows that evolution over time within one species and the branching off of a new species from an existing one (speciation) are two distinct but related processes. (Mayr himself played a central role in developing a fuller understanding of the speciation process.)
The second important aspect of Mayr's approach is his criticism of the fact that "most treatments of evolution are written in a reductionist manner in which all evolutionary phenomena are reduced to the level of the gene". This is directed against those like Richard Dawkins who see the gene as being the target of natural selection. Mayr shows that natural selection cannot "see" individual genes and that it is the organism as a whole which is the target of selection. It is the organism that lives or dies and succeeds or fails in reproducing, not the individual gene.
This last point has been reinforced by the fact that we now know that one gene can affect several different parts of an organism or its behaviour, and that several genes often interact with each other to produce one characteristic.
I strongly recommend this book. But I must point out that, although it is largely very clearly written, a few parts are quite difficult, especially for anyone who is new to evolutionary theory. For that reason I would recommend Jerry Coyne's book, "Why Evolution Is True" as a companion volume for newcomers.
Phil Webster.