GENERAL INTRODUCTION 3 



any sort, some of our living muscle is used up ; every time 

 we think or exert our will power, some of the living brain 

 substance is probably changed into dead waste material. 

 But in contrast to lifeless machines, our bodies are self-repair- 

 ing. The food we eat not only goes to increase the size of 

 the body ; it also furnishes material to make good the wear 

 and tear of everyday life. This power of self-repair is like- 

 wise present in all animals and in plants as well. 



4. Reproduction of Living Things. A third character- 

 istic that distinguishes living things from those that are life- 

 less is the fact that they produce seeds (in the case of plants) 

 or eggs (in the case of animals), which in turn come to form 

 plants or animals like those by which these seeds or eggs 

 were produced. No lifeless object can do this. We shall 

 find in our laboratory study that, while there are a great 

 many different methods of producing these new organisms, 

 still in their essential features these various methods of repro- 

 duction are much the same from the lowest plants to the 

 highest animals. 



5. Summary. In brief, then, we may say that all liv- 

 ing things have the power of growth from within, of self-repair 1 

 and of the reproduction of their kind; but that so far as we know 

 lifeless objects possess none of these powers. 



6. Science and its Subdivisions. Ever since the dawn of 

 history we find that mankind has been seeking to learn the 

 secrets of living and lifeless matter. During the past century 

 our knowledge has increased so rapidly that many sciences 

 have been completely rewritten. The discoveries, for ex- 

 ample, of the characteristics of radium and of X-rays have 

 revolutionized much of what was formerly believed as to the 

 properties of lifeless matter. In the same way our increased 

 knowledge regarding germs and other microscopic plants and 



