294 Biology in America 



watch, or to reflate our bites as we do our setting-up exer- 

 cises. In the feeding of hens for egg productivity Pearl has 

 shown that hens witli a mixed diet, from which they were 

 permitted to choose at will, maintained better health than 

 those limited strictly to certain articles. ''Eeason in all 

 things, excess in none," is a fundamental rule for sanity in 

 diet as in other of our life activities. 



What of the mechanism whereby this wonderful machine 

 of life utilizes its fuel? Herein lies one of the fundamental 

 differences between the living and the non-living machine. 

 "Whereas the latter uses its fuel solely in the conversion of 

 potential energy into heat and work, the former, in addition 

 to these two functions, also converts some of its fuel into its 

 own substance to take the place of worn-out parts, and to 

 build new parts and enlarge those already formed in develop- 

 ment and growth. We have already seen that the living 

 engine is much more efficient in the conversion of the potential 

 energy of its fuel into work than is the non-living machine. 

 How convenient it would be if the latter like the former were 

 automatically repaired as it wore out ! Given a good machine 

 to start with, proper fuel and draft, and preventing anyone 

 from throwing in dirt (disease) and the living machine will 

 run without repair for the time of its natural life. 



How is this done? In the non-living machine the process 

 of converting fuel energy into work energy is comparatively 

 simple. The carbon of the fuel is in such shape that it can'' 

 be more or less directly oxidized to carbon dioxide, and heat 

 energy thereby released. But in the utilization of the food 

 or fuel of the living machine a large number of intermediate 

 steps are necessary, which steps often consist of a cycle of 

 changes which are partly degenerative (breaking down com- 

 plex substance and thereby releasing energy) and partly 

 constructive (building up simpler into more complex sub- 

 stance and storing energy thereby). The food as taken into 

 the body of most animals is in such shape that it cannot be 

 directly burned to furnish energy^ or built up into body 

 substance. While our knowledge of the many complicated 

 changes undergone by food stuffs in the animal body is as 

 yet very meagre, we have nevertheless enough information to 

 enable us to follow in a general way these changes. Probably 

 the food most readily convertible into energy is fat. Some 

 fat is an exception to the statement made above that food 

 is not directly convertible into energy. The Esquimaux use 

 seal blubber both as food and fuel for heating their igloos, 



* I refer here to ordinary conditions of combustion. Any food sub- 

 stance may be burned in a special apparatus known as a bomb calorime- 

 ter and its energy content thereby determined. 



