144 THE STORY OF MILK 



growth of the young, which our present knowledge of 

 chemistry cannot explain. In Bulletin No. 17 of the 

 Wisconsin Experiment Station experiments with the 

 feeding of rats are described which show how butter-fat 

 could not be replaced in the ration by vegetable fats of 

 apparently the same composition and digestibility with- 

 out disastrous results, and similar conditions have been 

 found in regard to other foodstuffs, proteins as w^ll as 



The rat on the left got five per cent of cottonseed oil and the one on 

 the right got instead one and a half per cent of butterfat, otherwise 

 their rations were alike. These results are typical for any ration made 

 up of purified foodstuffs with butterfat in them as compared with any 

 fat of plant origin. The plant fats lack an unknown something with- 

 out which growth cannot proceed. 



fats. The yolk of eggs and butter-fat contain this un- 

 known something which is absolutely essential for the 

 growth of the child and which is missing in most sub- 

 stitutes, especially in lard and vegetable fats. 



The above illustration is from the work of Mc- 

 Collum and Davis at the Wisconsin Experiment 

 Station. 



Realizing the fallacy of the old rules for making up 

 rations for the feeding of farm animals, Professor 

 Evvard of Iowa is trying the reliability of the instincts 

 of animals as a guide to the proper selection of the most 



