416 DISEASE IN WILD MAMMALS AND BIRDS 
paralleled the advances in chemistry, especially the re- 
searches into the structural make-up of living cells, the 
intermediate stages in their upbuilding and degradation 
and the products resulting from their physiologi- 
cal activities. 
Incorrect feeding both qualitative and quantitative un- 
doubtedly plays an important role in producing disease. 
In the early works on nutrition, the proportion of fats, 
carbohydrates and proteins was regarded as the essential 
point of a normal diet. The researches on the composi- 
tion of foods marked the first real epoch in this history 
and Fischer's (1) studies on the variation in the composi- 
tion of proteins from different sources first introduced 
the idea of quality. Later Mendel and Osborne investi- 
gated the biological values of purified proteins, while at 
the same time McCollum and others were studying the 
value of the groups of proteins oc<jurring in a single 
natural food stuff, were calling attention to the so-called 
vitamines, and were emphasizing the need of balanced 
inorganic materials. These studies have practically 
revolutionized our knowledge, particularly of the effects 
of badly balanced foods. They have clearly demonstrated 
that dietary values can, in all probability, be discovered 
only by careful biological study of feeding experiments 
together with the finer analysis of the components of the 
diet, especially of the protein and fat radicles. At the 
same time a definite appreciation of the role of each ele- 
ment in metabolism must be kept in mind. 
These varied studies on nutrition have shown that the 
chemical requirements of a diet are in their ultimate 
analysis essentially the same for all species of the higher 
animals — that is all require approximately the same 
amount of protein, fat, carbohydrate, etc., per kilo of body 
weight, while the morphology of the tract decides the 
physical properties of the diet. 
(1) Chemistry of the Proteins, Mann. 
