86 COMPARATIVE PHYSIOLOGY 



intake of a definite amount of oxygen ; and that the ratio of 

 carbon dioxide evolved to oxygen used up is different for 

 carbohydrates, fats, and proteins respectively. This ratio is 

 known as the respiratory quotient. It is, as would be expected, 

 unity in the case of carbohydrates, and less than unity for 

 fats (071) and proteins (cyS). The investigations of Liebig 

 coincided with the formal statement of the conservation of 

 energy by Mayer on the basis of Joule's determination of the 

 mechanical equivalent of heat. The general applicabilit}'- of 

 the first law of thermodynamics to living organisms was 

 universally accepted as the basis of physiological research by 

 the end of the nineteenth century. 



For the greater part of this period, however, the impossi- 

 bility of oxidising animal foodstuffs at such temperatures as 

 are consistent with organic existence as we know it, or of 

 stimulating the digestive reactions in vitro without recourse 

 to reagents which would be fatal to the organism, presented 

 an inflexible barrier to the probability that the mechanism of 

 living organisms conforms to the known laws of energetics. 

 To-day the position has changed in two ways. The study of 

 those more complex chemico-physical systems which are for 

 convenience described as *' colloids," and the role of surface 

 tension, osmotic pressure, and electrolytic dissociation in 

 modifying their properties, opens up a new horizon of possi- 

 bilities, while the extension of the principle of catalysis to 

 enzymes, and its clarification by Ostwald and others, has 

 thrown a flood of light on the chemical equilibrium of the 

 organism. 



Sources of Animal Food. — Fats, proteins, and carbohydrates 

 are the principal constituents of a healthy diet. There is a 

 certain amount of evidence that fats and carbohydrates 

 are convertible into one another, and that carbohydrates can 

 be manufactured from the deaminised products of protein 

 metabolism. Protein as such is not necessary ; but it seems 

 that it can only be replaced by its hydrolysis products, the 

 amino-acids. In this connection some interesting bionomic 

 problems arise. 



It was found by Loeb (19 15) that the banana fly (Droso- 



