Organisms are adapted by natural selection to their physical and biological environments. Individuals differ within and between species in, among other things, such features as their body size, their age at first reproduction and the effort they put into growth and reproduction. The body of theory, generally known as life-history theory, that considers such features is extensive and growing rapidly (Stearns, 1980; Calder, 1984; Sibly & Calow, 1986). In a seminal review of these theories, and of the data that support them, Stearns (1976) concluded that we need more comprehensive theory that makes more readily falsifiable predictions. This view has been echoed by others (e.g. Peters, 1983). One way of viewing this book is to see it as presenting a number of life-history theories. I have tried to make the theories comprehensive, to make their assumptions explicit, to investigate the sensitivity of their conclusions to variations in the assumptions, and to test them whenever possible.

In common with most spheres of knowledge, the field of allometry has expanded very considerably within the last ten to fifteen years. It is perhaps no longer possible for a single text to provide a detailed review of all the work (see Peters, 1983; Calder, 1984; Schmidt-Nielsen, 1984). I hope this book has a single thread that runs through it, but I have tried to restrict myself to those areas where I feel I may have something new to contribute.

In essence, this whole book rests on two equations introduced in Chapter 2. These equations relate the scaling of average daily metabolic rate and energy assimilation intraspecifically to body weight, W.