The last few years have seen a number of new books on evolutionary biology. However most of these are either large or specialized. This is an attempt to produce a thin, general version for undergraduate use. Thinness, of course, demands selectivity, and the aim has been to concentrate on the principles of the subject rather than on the details-principles, that is, of both theory and practice. Thinness also sometimes means that a certain level of knowledge is assumed in the readership, but I hope that this is not the case here, and my intention has certainly been to produce something that is as intelligible to the uninitiated as it is to the well-informed. As for the bibliography, I refer, where possible, to reviews rather than primary sources, so a citation should not be taken to imply any sort of precedence. In developing the theme, I have adopted a loosely historical approach, not only because I believe that this makes for more interesting reading but also because the subject, like the subject it addresses, has evolved under the critical eye of a selective process. Problems have been perceived, hypotheses have been formulated to explain them, facts have been amassed to test the hypotheses, more problems have been perceived, more hypotheses formu lated, and so on.
