FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 1 5 



not be made for the perversions of function thereby introduced. 

 For this reason we have been inclined to lay stress upon only those 

 experiments which were either successful or which failed because 

 of obvious causes. 



In selecting criteria of adequate growth the painstaking statis- 

 tical studies of Donaldson* on the adjustment of size to body-weight 

 and age in the white rat have been of great help. After birth the 

 young white rat depends upon the mother for sustenance for about 

 20 days. The span of life is about three years. Sexual maturity 

 is reached in about 60 days. The first year of rat life corresponds, 

 according to Donaldson, to the first thirty years of human life; and 

 the growth curve for this period has been published by him. Some 

 of the details are reproduced in Chart I. 



The lack of appreciation of the salient features of these curves, 

 representing graphically the gross normal increase in weight of white 

 rats during the first third of their life, has led occasionally to con- 

 clusions which appear to us as quite erroneous. If, for example, a 

 rat weighing 250 grams maintains its body- weight for several weeks 

 without marked variations one may properly conclude that a normal 

 nutritive equilibrium exists in such an animal ; on the other hand a 

 rat whose initial weight is 70 grams is in a period of most active 

 growth. Normal nutrition for an animal in this phase calls for a 

 measurable daily increment in weight and a gradual, yet detectable, 

 increase in body-length. Within one month a 70-gram white rat 

 ordinarily will double its weight when the diet is adequate. The 

 illustrations cited suffice to indicate how different must be our cri- 

 teria for the adolescent and the adult stages. 



*Donaldson: A comparison of the white rat with man in respect to the growth of 

 the entire body. Boas Memorial Volume, 1906. 



