38 MICHIGAN ACADEMY OF SCIENCE. 



that cannot be measured or brought under control, so that it is impos- 

 sible to determine their relative or combined influence on the results ob- 

 tained. 



The appetite and previous habits of the animals consuming the food, 

 the amount eaten, and the efficiency of their organs of nutrition in per- 

 forming the work of digestion and assimilation must be recognized as 

 modifying factors that are quite as important in determining nutritive 

 values as the composition of the food itself, and it is evident that experi- 

 ments cannot be repeated under the same precise conditions for the 

 purpose of verification. 



The chemical composition of foods cannot be made to represent their 

 nutritive value, as there are physical and biological factors that are quite 

 as significant. Liebig's false theory that the nitrogenous constituents 

 of foods (proteids) were exclusively used in the building of tissues, and 

 that the non-nitrogenous constituents were burned in the system to pro- 

 duce animal heat, has been a fruitful source of error in planning and 

 conducting feeding experiments. Carbon and oxygen and the ash constit- 

 uents of food are quite as important factors in tissue building as nitro- 

 gen to which attention is almost exclusively directed, and food constit- 

 uents are not burned in the system to produce animal heat. 



The law of the conservation of energy is as strictly observed in organic 

 processes, as in the reactions of inorganic matter, and the transforma- 

 tions of energy in the economy of living organisms are now attracting 

 the attention of physiologists as the most significant results of the meta- 

 morphoses of matter. Work must be done in the building and repair of 

 tissues, and the energy so used, derived from the food consumed, is 

 stored up in the organic substances formed. In the destructive metabol- 

 ism that follows from the wear and tear of tissues in their functional 

 activities, this stored energy is liberated and what is not immediately 

 required in the constructive processes of the system appears as animal 

 heat. 



No general statement in regard to the nutritive value of foods can be 

 formulated from the results of experiments in which the chemical factors 

 are alone considered and Liebig's classification of foods has not the phy- 

 siological significance claimed for it. The same animal may give quite 

 different results with the same food at different times, and different ani- 

 mals are not likely to agree in the returns given for the same food under 

 the same conditions. 



From the complex processes of soil metabolism and the various con- 

 ditions that have an influence for good or ill on the well being of the 

 plants themselves, and the micro-organisms concerned in the elaboration 

 of plant food it may be readily shown that the sources of fallacy are quite 

 as evident in field experiments as in the feeding of animals. In both 

 cases the farmer is dealing with living organisms that thrive best when 

 fully satisfied with the conditions in which they are placed. In nearly 

 all problems that arise in these departments of his calling the farmer 

 will be best aided by researches in pure science for the increase of knowl- 

 edge relating to the facts and principles of biology. 



