10 INTRODUCTION 



salts are so essential to life that if they are withheld from the 

 animal body, death ensues much more quickly than from the lack 

 of proteins, carbohydrates, and fats. In solution the salts of the 

 body provide the proper medium for living tissues, and aid in main- 

 taining the optimum condition of physiological equilibrium. (For 

 a further discussion of the significance of the salts of these elements 

 see page 437.) 



Enzymes are complex organic substances produced in living cells 

 by glands some of which are modified to produce other important 

 secretions. Enzymes are able as catalysts to hasten chemical reac- 

 tions, but do not enter into the end product of the reaction, and are 

 not themselves consumed. 



Endocrine glands are glands of internal secretion that produce 

 catalyzing hormones which act as "messengers of stimulation," in- 

 fluence growth, and regulate the animal througlfHlyMjj^ole life. 

 (See page 444.) 



Vitamins are certain accessory food factors that have been 

 studied extensively since 191 1. Just what their chemical com- 

 position may be has not yet been fully decided. The reader will 

 find further discussion of the subject in the section on Nutrition and 

 the Endocrines (page 441). 



Animal Relationships 



Fitness to Survive. — If we study simply the structure and the 

 activities of organisms and neglect the relation of the living being 

 to its environment, our study is not Biology, or the science of life. 

 Biology introduces a new problem, that of fitness. Biology asks 

 "When, how, why.?" Aristotle said: "The essence of a living thing 

 is not what it is made of, nor what it does, but why it does it." 



It is the adjustment of the individual to his environment which 

 makes life possible. Life is one long continuous fight, a "struggle 

 for existence." T. R. Malthus in his "Principles of Population" 

 showed that human population tends to increase in a geometrical 

 ratio (i, 2, 4, 8) while the food supply tends to increase in an arith- 

 metical ratio (i, 2, 3, 4, 5, 6, 7, 8). Famines are still occurring in 

 India and China. 



Today man is dependent on his fellow man for his subsistence 

 and directly or indirectly dependent on the plants and animals 

 about him. Each animal or plant represents a force in Nature and 

 is beneficial or injurious. If we study the habits of animals we will 



