The third case mentioned, viz., that in which the nutritive equilibrium is exactly maintained, so that the body weight remains unaltered, is the most important one for us to determine, since its final settlement would enable us to fix the most beneficial standard of diet. Unfortunately, this case is also the most difficult upon which to come to a satisfactory conclusion, for the following reasons: -

1. The elaborate nature of the conditions imposed during the experiment makes it difficult to carry on the investigation with scientific accuracy.

2. Even when the amounts of gain and loss exactly correspond we cannot say that we have the best dietary; because some of the income may be quite useless, and pass through the economy without performing any function, and yet appear in the output so as to give an accurate balance.

3. We have just seen that the relative amounts of outgoings and of material laid by as store are altered and regulated by the quantity of income. And we find that the quality of the income, i. e., the relative proportions of the various food stuffs, has a material influence on the quantities of material laid by and eliminated respectively. We must, therefore, consider the efficacy of each of the groups of the food stuffs when employed alone and mixed in different proportions.

4. Different animals seem to have different powers of assimilation; and under various circumstances the requirements and assimilative power of the same animal may vary.