HOW PLANTS AND ANIMALS GROW. 513 



animals, arising from individual peculiarities and inherited an- 

 cestral habits. Experiments have not as yet been made with a 

 sufficient number of individuals or with a sufficient variety of 

 foods to warrant any generalizations as to what constitutes a 

 normal diet for either man or beast under even average condi- 

 tions. 



The recent recognition of energy as one of the most important 

 factors in physiology has led to the rejection of the purely chem- 

 ical theories that were formerly quite generally accepted in regard 

 to the role of particular food constituents in the processes of nu- 

 trition. An assumed combustion of food constituents is no longer 

 required to explain the phenomena of animal heat, which is noV 

 known to be but a phase in the transformations of energy in the 

 processes of nutrition. 



Energy is expended in building organic substance, or, in other 

 words, in converting food-stuffs of any kind into protoplasm, the 

 summit of the double stair of life, and its potential energy is the 

 transformed or stored energy of the constructive process. This 

 combined energy, in accordance with the law of conservation, 

 may be liberated in the form of heat to a greater or less extent in 

 various ways by the more or less complete disintegration of the 

 organic substance in which it is stored. If the process of disin- 

 tegration is carried on until the organic substance is resolved into 

 its original elements, the heat liberated is the exact equivalent of 

 the energy expended in its construction. 



In living organisms the descending steps of metabolism are 

 but successive phases of normal vital activities, resulting in the 

 formation of a definite series of organic substances which contain 

 less potential energy than the protoplasm from which they are 

 formed, and heat must therefore be liberated as they are elabo- 

 rated. Dead organic matters may be torn apart by microbes and 

 resolved by a widely different series of descending steps into their 

 original elements, as in the processes of fermentation and putre- 

 faction, with a complete transformation of their potential energy 

 into heat. The same ultimate result may likewise be obtained by 

 burning organic substances, but the intervening steps and prod- 

 ucts of the destructive process are less numerous and of a dif- 

 ferent character than those produced by vital activities, while the 

 heat liberated is still the transformed energy of the constructive 

 process. 



Plants derive the energy required to convert simple chemical 

 elements into the complex molecules of protoplasm from the heat 

 and light of the sun, while on the other hand the energy expend- 

 ed in the constructive processes of animals is exclusively derived 

 from the potential energy of their food (stored energy of plants), 

 and a disintegration and apparent waste of the material, or chem- 



VOL. XLIII. 37 



