INTRODUCTION 3 



In the present state of scientific knowledge it is impossible to answer 

 the question " What is life ? " though it is possible to describe those character- 

 istics which distinguish the living from the non-living. In living matter we 

 find only those elements which are among the common constituents of the 

 earth, and these elements obey the ordinary laws of Chemistry and Physics. 



All living organisms are built up of a substance known as protoplasm. 

 This may be termed " living matter," or, in the words of the great zoologist, 

 T. H. Huxley (1825-95), " The physical basis of Hfe." One of the most 

 striking features of living organisms is that their protoplasm is never stable, 

 but is constantly undergoing change. All changes in matter imply the 

 performance of work, which in turn can only be carried out by the use of 

 energy. Every activity, therefore, on the part of a living organism demands 

 the expenditure of energy, and it is for the purpose of obtaining this energy 

 that many of the vital functions are performed. 



Though some of this energy is dissipated, usually as heat, much is utilized 

 in the manufacture of fresh material to take the place of that which has been 

 broken down. The sum total of this breaking down and building up again 

 is spoken of as metabolism. This cycle is not perpetual, however, since 

 there is an inevitable wastage of energy which must be made good from 

 without. Thus every organism requires an external source of energy to keep 

 its metabolism going. 



In adult life these two processes are fairly equally balanced. If building- 

 up is in excess of breaking-down, as in young organisms, there is a definite 

 increase in the amount of protoplasm, resulting in growth. On the other 

 hand, if the reverse should occur, as is common in old age, there is a gradual 

 increase of destruction over repair, which will finally result in death, though 

 for various other reasons death may occur even when large quantities of 

 food reserves are present. 



Growth, of course, may be observed in non-living matter. A familiar 

 example is the growth of crystals in the chemical laboratory. But this kind 

 of growth is by accretion, in which layers of new molecules of the same kind 

 are superimposed upon the existing ones. It difi'ers fundamentally from the 

 type of growth found in organisms, where the fresh molecules, not necessarily 

 of the same kind, are interspersed among those already present, and trans- 

 formed in such a way that they can be built into the substance of the organism 

 throughout its body. This type of growth is characteristic of living organisms, 

 and is known as intussusception. 



Break-down is essentially a process of oxidation, as a result of which 

 oxygen is used up and carbon dioxide is formed. This gaseous interchange, 

 with the liberation of energy, constitutes the essential feature of respiration. 

 All living organisms, whether animals or plants, continuously respire through- 

 out life. The chief sources of energy are carbohydrates, such as sugars, which 

 are found in the protoplasm and are easily broken down by respiration. 



Occasionally substances are formed in the course of metabolism which 

 are put to some special use outside the usual metabolic cycle. These sub- 

 stances are termed secretions. 



