82 THE AMINO ACIDS 



those only can hope to reach it who keep the summit 

 constantly in view. All our labor and all our efforts, 

 if we strive to attain it through a morass, only serve 

 to cover us more completely with mud; our progress 

 is impeded by difficulties of our own creation, and at 

 last even the greatest strength must give way when so 

 absurdly wasted." 



The development of knowledge in science succeeds 

 best when an hypothesis is formulated as a basis for 

 investigation. By holding fast to that which is proven 

 as fact and discarding that which is shown to be con- 

 trary to fact is real progress made. This, indeed, has 

 been the case in the history of protein metabolism, as 

 may be seen in the following pages where is traced the 

 evolution of ideas concerning it. 



LIEBIG 



The first clearly defined theory of protein metabo- 

 lism was that enunciated by Liebig who assumed that 

 protein material undergoes little or no chemical change 

 previous to its introduction into the blood stream and 

 its assimilation by the tissues. "According to this 

 theory, the plant holds a position intermediate between 

 the mineral and animal world. The animal is incapable 

 of assimilating the compounds stored up in inorganic 

 nature. To render these compounds subservient to 

 the purposes of animal life they may have to undergo 

 a preliminary preparation within the living organism of 

 the plant. The simple mineral molecules are thus con- 



