A distinction must be made between the ontogeny and the phylogeny of metabolism in order to understand the mass exponent of energy metabolism.

The discussion on the metabolism/body mass relationship in animals has been complicated, almost from the beginning, by a confusion between intra- and interspecific comparisons of energy metabolism. In this paper I argue that the ontogeny of metabolism is a complex process in which several phases can be distinguished, each with a specific metabolism/body mass relationship. In vertebrates at least the longest of these phases, encompassing the best part of adulthood, appears to follow the 'surface rule' of metabolism. According to this rule aerobic power (En) can be described by the equation En = an X M0.66 in which 'an' is the specific mass coefficient which itself depends on adult body mass according to the equation an = a X M0.09. Since, in a series of related species, the specific mass coefficient, an = En/M0.66, increases with body mass in a systematic manner, it follows that adult metabolism of these species can be described by a second function, E = a X M0.75, in which 'a' is the interspecific mass coefficient (3.34 W in mammals if M is in kg). This function represents the phylogeny of metabolism which can be interpreted only within an evolutionary context and not by introducing new parameters into Rubner's 'surface rule'.