Discussion of this topic requires clear definition of terms. For instance, because size can be such an ambiguous word, it is essential at all times to state the units of measurement of size of the animal. The units may referto height, weight, skeletal size, metabolic body weight or to some component of body composition. Most frequently the subject is discussed in terms of body weight, or some function of it representing metabolic body size, and little cognisance is taken of possible differences in body composition. I consider this to be a serious omission, because it seems highly unlikely that weight of the animal will have the same relationship to productivity irrespective of the composition of the weight.