Metabolism 57 



growth of a non-living crystal or sand pile. There is first of 

 all the fact that whereas a crystal or sand pile grows by the 

 addition of similar material, the plant or animal takes from 

 its surroundings entirely different materials and converts 

 them into its own substance. This process of changing for- 

 eign substance into itself, which so far as we know is to be 

 observed only in living bodies, is called assimilation, or mak- 

 ing alike. Moreover, in a living plant or animal, this assimila- 

 tion takes place throughout the whole living structure and 

 not merely upon the surface. The baby, for example, takes 

 in materials derived from a cow, from a wheat plant, or from 

 a potato plant, and converts these materials into what is at 

 last indistinguishable from baby stuff. There is an increase 

 of baby stuff at the expense of these foreign substances which 

 we call food. And exactly the same kind of process takes 

 place in all living things. This assimilated material brings 

 about growth, not by a mere external addition, or by a filling 

 in as one would cause a mail pouch to expand by stuffing 

 more into it, but by enlarging each of thousands or millions 

 of tiny units of which the organs are made up. 



Furthermore, whereas the growth of a crystal or any 

 other non-living body can continue as a result of a steady 

 supply of suitable material, the growth of a living animal or 

 plant involves not merely the appropriation of suitable 

 " foods " but also a steady output of waste substances. That 

 is to say, there is involved in the assimilating process a large 

 number of chemical activities which bring about both the 

 formation of living substances out of the income or food, and 

 the creation of a large number of substances that are not used 

 by the living organism and are discharged to the outside. 



Metabolism 



The totality of these many chemical changes that go on 

 within living substances is called metabolism^, that is, " work- 

 ing over "; and it includes the building up of living matter 

 and its breaking down. 



