406 ANNUAL REPOT?T SMITHSONIAN INSTITUTION, 19 31 



It must not be thought, however, that numerous adaptations of 

 the organic portions of milk to the needs of the young animal can 

 not be shown. The organic constituents of milk may be divided into 

 those which can only be used as fuel and those which furnish material 

 for the construction of the body of the young animal. If we compare 

 the milks of animals whose young grow at different rates, for ex- 

 ample, we find that there is a definite relation between the concentra- 

 tion of the organic building materials and the rate of growth, the 

 concentrations being greater in the milk of the more rapidly growing 

 animals. A similar relation, incidentally, is to be seen between the 

 rate of growth and the total concentration of the inorganic con- 

 stituents. 



Even the constituents of the milk, which serve only as fuels, al- 

 though they play no part in the contribution of material for build- 

 ing up the body of the young animal, show adaptations to the vari- 

 ous needs, not merely of different species, but even of different indi- 

 viduals. 



The young of warm-blooded animals living in cold climates are, for 

 example, exposed to greater losses of heat than those of animals liv- 

 ing in warmer regions. We find a correspondingly higher concen- 

 tration of the fuel, fat, in the milk of the animals indigenous to cold 

 regions. Again, other things being equal, small animals tend to 

 consume relatively greater quantities of energy per day than larger 

 animals. The milk of small animals is, in general, richer in fat 

 than the milk of larger animals. This relation can not only be 

 seen among different species of mammals varying widely in size, 

 but even in individuals of the same species. The small Jersey cow, 

 for example, yields a milk richer in fat than larger breeds. 



The range of variation of size among human individuals is much 

 smaller. The correspondences between the composition of the milk 

 and the needs of the human infant are, therefore, much less obvious. 

 They require a closer scrutiny for their discovery. But it may be 

 shown, by suitable methods, that there is a definite correlation be- 

 tween relatively slight variations of the physical characters of 

 healthy infants and the composition of the milk with which they 

 are supplied by their mothers (unpublished observations). This is 

 surely a striking example of the length to which the adaptation of 

 the immediate environment to the needs of the organism is carried 

 by the most highly organized of all animals. 



(B) EXCHANGES OF ENERGY 



None of the devices to which the living organism resorts for the 

 control of the composition of its immediate environment can exert 

 its full effectiveness if the temperature of this environment is 



