432 SCIENCE PROGRESS 



His difficulties arise largely from the patent incompleteness 

 of physiological theory on the one hand, on the other from the 

 manifold imperfections of the methods commonly used in deter- 

 mining the content of utilisable nutrient matters in foods. 



During many years past it has been the common practice to 

 compare the merits of different foods or rations in terms of 

 their content of protein, fat, carbohydrates and "fibre," without 

 taking into account any quantitative differences in the make-up 

 of the materials comprised under these designations. It has 

 further been the custom to insist in the case of each class of 

 stock upon a definite " balance " being maintained between the 

 protein and non-protein constituents of the ration (" albuminoid 

 ratio") as a matter of fundamental importance. Fats, carbo- 

 hydrates and any excess of protein beyond the indispensable 

 minimum have been regarded as mutually interchangeable in 

 the proportions of their " isodynamic equivalents." The appli- 

 cation of these views to farm practice, however, has met with 

 overwhelming difficulties from the start. There are difficulties 

 which any system will meet with necessarily, such as the great 

 variability in the composition of the foods that form the staple 

 of the ration and the individual variations in feeding capacity 

 between different animals. But even in cases in which these 

 general difficulties have been largely overcome, there has often 

 been a hopeless discordance between theory and practice. 

 Rations esteemed, from theoretical considerations, to be of equal 

 value, have frequently given widely different results in practice. 

 Albuminoid ratios condemned outright by theory have in 

 innumerable cases proved in practice to be in no whit inferior 

 to the optimum ratios of theory. 



In the main, doubtless, the method has served the useful 

 purpose of correcting gross errors in feeding but its application 

 is so uncertain that it has never won the confidence of the 

 skilled feeder and voices have not been wanting to suggest 

 that theory has as yet little or nothing to offer in the way of 

 guidance to experienced practice. 



Some explanation of the discrepancies has been suggested 

 by the results of recent research on nutrition. We realise now 

 clearly that all proteins are not to be treated as mutually 

 equivalent and that " amides " need often to be taken seriously 

 into account. We know further that the attainment of the full 

 nutritive value of certain foods is conditioned by the presence 



