620 JOHN R. MURLIN 



speed in relation to energy intake in several genera and orders of mam- 

 mals. A kilogram of body substance in several of them contains, accord- 

 ing to Rubner(cc), 30 gm. N and 1722 calories of potential energy. To 

 produce this unit of growth requires in the earliest period of postnatal de- 

 velopment approximately the same amount of food energy ; namely, 4088 

 calories. The human infant, however, occupies an exceptional position, 

 in this regard, which may be expressed as follows. Of 100 calories of en- 

 ergy in the form of milk there is utilized for growth in the 



Colt 33.3% 



Calf 33.1% 



Lamb 38.2% 



Pig 40.2% 



Puppy dog 34.9% 



Kitten 33.0% 



Young rabbit 37.7% 



Average 34.3% 



Human Infant , 5.2% 



The average ingestion of milk in relation to the maintenance require- 

 .ment (this term in Rubner's usage is not synonymous with basal metabo- 

 lism) in the mammal is 202 per cent, while for the infant it is only 120 

 per cent. 



The relatively long infancy period in the human family, it would seem, 

 is a consequence rather than a cause of this difference; for if the large 

 amount of time spent in sleep explained the low intake of food, and the 

 slow development were a consequence of this, then keeping the baby awake 

 and thereby increasing the demand for food ought to accelerate its growth. 

 Of course just the opposite is true. Owing to a growth impulse of low 

 speed, which in turn probably determines capacity for food (anatomical 

 capacity of the stomach and functional capacity of metabolism) on the 

 part of the infant, the human mother is called upon to supply intelligent 

 care and protection rather than bulk of nutrients. Interesting biological 

 implications are involved which space does not permit us to develop at this 

 time. 



It is doubtful whether the growth quota of energy, i. e., the portion 

 left over after the maintenance factor, the activity factor, the dynamic fac- 

 tor and the loss by non-absorption have been covered, can ever be fixed 

 as a definite percentage of the maintenance metabolism for all varieties of 

 infants. The growth impulse, as between individuals, quite as truly as 

 between different orders of animals, is more a matter of heredity than 

 of food. Moreover, it is inherited from the father equally with the mother, 

 so that a small mother nursing the child f a large father may not be able 

 to supply milk enough for the rate of growth which the child has inherited. 

 Again it is well known that growth in height often will proceed at a time 

 when nutrition is not sufficient to support growth in weight, and both vary 

 with the season of the year (Porter, Bleyer). In time we shall have in 



