SOURCES OF KNOWLEDGE 215 



thus made possible and the necessity for theoretical 

 assumptions is largely removed. 



301. Study of the efficiency of individual proteins. — 

 The type of investigation in animal nutrition upon which 

 much emphasis is now placed is a study of the function 

 and relations of individual compounds. The proteins 

 have been especial objects of studies of this kind, studies 

 which have been made possible through the isolation and 

 identification of many individual vegetable and animal 

 proteins. The general plan of work has been to feed only 

 one well-identified protein in connection with a sufficient 

 supply of all the other nutrients necessary to maintenance 

 and growth. In this way it has been demonstrated that 

 the individual proteins are greatly unlike in their nutritive 

 value and relations, it being found that some will not 

 sustain growth or even maintenance, while other single 

 proteins constitute an efficient protein-supply for any 

 known protein use. 



It is aheady made clear to the reader, doubtless, that 

 the demonstration of facts and principles in the domain 

 of animal nutrition is exceedingly difficult. It should 

 be equally clear that when conclusions are reached in ways 

 which have been briefly described, they are worthy of 

 respect and should have greater weight than the neces- 

 sarily imperfect observations of common practice. Science 

 often errs in her deductions, but the efforts of her workers 

 are constantly directed toward the elimination of false 

 conclusions, so that unsound theories are not likely to be 

 accepted for a great length of time. 



