276 SCIENCE REMAKING THE WORLD 



protein-free milk an artificial mixture built of the salts 

 in the proportions indicated in the analysis, it failed to 

 produce the same effect. 



Stepp, a worker in Germany, had been attracted to 

 the problem by the researches in protein quality and 

 on the hypothesis that fats also might differ in quality 

 experimented along this line with rats. He first demon- 

 strated that bread and milk constitute a growth- 

 producing diet for rats. He then extracted his bread- 

 and-milk mixture with ether and found the residue 

 inadequate for growth. This result seemed confirmatory 

 of his viewpoint. But when he added to the residue 

 purified fat which he assumed was what had been re- 

 moved by the ether extraction, no growth resulted. 

 On the other hand, the residue obtained by evaporation 

 of his ether extract when mingled with the other residue 

 produced normal growth. Stepp failed to grasp the 

 entire significance of these experiments at the time, but 

 he did provide additional evidence that milk contains 

 something that is neither protein nor fat and which is 

 essential to growth. 



In 1906 an experiment was begun at the Wisconsin 

 Experiment Station which was planned by S. M. Bab- 

 cock and carried out by Hart and Humphrey. In the 

 later stages of this experiment McCollum and Steenbock 

 cooperated. The object of this experiment was to 

 determine whether rations for cattle, so made up as to 

 be alike so far as chemical analysis would show, but 

 derived each from a single plant, would prove to be of 

 equal nutritive value for growth and the maintenance 

 of vigour. The plants selected were wheat, corn, and 



