Ecologists generally place more importance on science as explanation than science as prediction. In this book, that emphasis was reversed by stressing the predictive and descriptive roles of theory. Nevertheless, one cannot but wonder why the power formula, in general, and the mass exponents of ¾, ¼, and −¼, in particular, are so effective in describing biological phenomena. This chapter addresses such questions by examining some of the explanations proposed for the ¾ law. I do not consider the problem solved and so offer only a review without conclusion.

Two basic components of allometric explanations

Similitude

One of the striking features of allometry is that very different processes show parallel responses to variations in body size. This parallelism is usually referred to as the principle of similitude (Thompson 1961) or similarity(Kleiber 1961). This may be stated in a restricted form: For example, the size relations of biological rates can be described by a power formula in which the exponent of mass is approximately ¾. A wider formulation could be used instead: Over the longer term, the ratio of physiological processes is constant. The second formulation is the more testable for it applies wherever a range of rates are encountered. The first is, however, a more accurate description of the empiricisms on which the principle is founded. Either statement is empirically based and should be viewed as a general theory.