INTRODUCTION 3 



are asked to believe that this constancy, so evident to our 

 experience, is a delusion, and that change is the rule in the 

 animal world. If we come to think of it, change is the rule, 

 even among individuals. The individual man passes from 

 infancy to childhood, from childhood to youth and manhood, 

 and finally to senility and death. All these stages in his 

 career mean change, and we recognise the same changes as 

 occurring in animals. Further reflection assures us that even 

 during the stages when the body seems to have arrived at a 

 condition of stability of some duration, there is daily change 

 going on. A man gains or loses in weight according to the 

 amount of work he does and the amount and form of nourish- 

 ment that he takes. It is a familiar fact that if we would 

 do an increased amount of work we must take a larger amount 

 of food, that if we do less work we must take less food, or pay 

 the penalty of becoming stout. The same thing applies to 

 the animals which we domesticate and compel to work for us, 

 and though we cannot follow the steps so closely in the case 

 of wild animals, we have no doubt that they too are subject 

 to the same conditions. 



In short, animal life is the outcome of a never-ceasing series 

 of chemical changes. An animal is constantly taking in 

 substances as food, converting these substances into the living 

 material of its own body, and then, when it exhibits energy 

 of any kind, this living material undergoes chemical change, 

 is broken down into substances of comparatively simple 

 chemical constitution, which, being of no further use in the 

 body, are forthwith expelled as excreta. 



This condition of constant exchange of material is character- 

 istic of all living things, whether animal or vegetable, and marks 

 them off sharply from dead or inanimate objects. A living 

 organism is constantly manifesting energy, whether in the form 

 of motion or of heat. The source of this energy is chemical 

 change in the constituents of its body. These constituents, 

 of complex and unstable chemical ' constitution, enter into 

 combination with the oxygen of the air, are broken down into 

 substances of less complex and more stable constitution, which 

 are ejected from the body, and if the loss thus occasioned 

 were not made good the body would waste away. The loss 

 is made good by food, which is converted into new living 

 tissue to be broken down afresh ; and so the process goes on. 



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