GROWTH 391 



energy (463, 464) in the young animal as compared with the 

 more mature one. Is this difference to be ascribed to a specif- 

 ically higher percentage utilization on the part of the younger 

 animal, or is it due to a relatively greater consumption of feed 

 or the relatively high net energy values which usually char- 

 acterize the feeds given the young animal, particularly milk? 

 The mere fact, for example, that a young animal consuming 

 milk utilized a higher percentage of the feed energy than did 

 the same animal later upon a mixed ration would not necessarily 

 show any physiological superiority on the part of the younger 

 animal but might be due solely to the difference in the kind 

 of feed consumed. So, too, the mere ability to consume 

 relatively large amounts of highly concentrated feed in the 

 form of milk and thus to secure a large surplus above the 

 maintenance requirement might (360, 510) give the younger 

 animal a marked economic advantage without indicating any 

 more efficient conversion of the surplus energy supplied than 

 in the older animal. 



Unfortunately, investigations regarding the utilization of 

 feed at different ages have been few in number and the avail- 

 able data regarding net energy values for growth are exceed- 

 ingly meager. To a large extent it is necessary to be content 

 with comparisons of a very general nature, leading to proba- 

 bilities only. 



473. Experiments on suckling animals. The experiments 

 by Soxhlet on calves and those by Wilson on pigs cited on 

 previous pages (458) and likewise an investigation by Rubner 

 and Haubner 1 on infants afford some data for approximate 

 estimates of the percentage of the metabolizable energy of milk 

 utilized by growing animals. 



The computations involve a number of uncertain assump- 

 tions, particularly as regards the maintenance requirement, 

 and none of them afford a satisfactory basis for comparing 

 the utilization of the metabolizable energy of milk at different 

 ages. It is of some interest, however, to compare the average 

 utilization computed from these experiments with that esti- 

 mated by the use of Rubner's factors for the " specific 

 dynamic action " of equal amounts of pure nutrients on mature 

 animals. 



l Ztschr. Biol., 36 (1898), i ; 38 (1899), 315. 



