It is a characteristic feature of elementary biology texts (e.g., English “A-level” texts such as Roberts 1987, Simkins and Williams 1989) that they almost always include their one or more chapters on evolution near the end of the book. It is traditional to include a section on “evidence for evolution”, with the principal evidence cited as from Comparative Anatomy, Embryology, Physiology, Biochemistry (including immunology), and sometimes Behaviour. But all these are evidence for the phenomenon of natural classification. The fossil record may be discussed, but more as an abbreviated history of the biosphere than as a basis for a logical argument, and there is usually a brief treatment of the evidence from geographical distribution; but the missing ingredient is any clear statement of the scientific reason for proposing a theory of evolution.

One of the most important points to be emphasised in this book is that phylogeny is the theory proposed to explain natural classification. The pattern of evolution, or phylogeny, and particularly common ancestry, is the explanans; the phenomenon of natural classification is the explanandum (Brady 1985). In other words, the theory that evolution has occurred is the theory that the apparent relationships of classification are real relationships – that is, the pattern of classification is in fact a family tree.