contraction, excretion, and so on, metabolism could not remain constant; It 

 cannot just "keep on". 



For being alive means something more than the sum of all these processes 

 which we observe in organisms. Each detail of action depends not only upon 

 the outside conditions; it depends upon all the other processes. And the re- 

 lationships among these processes are always changing. Assimilation, for exam- 

 ple, depends upon absorption. The rate of oxidation depends upon the tem- 

 perature as well as upon supplies of oxygen and of fuel. Metabolism depends 

 further upon the removal of wastes, but this in turn depends upon the relative 

 concentration of substances inside the cell and outside it. No process goes on 

 by itself. 



Even in so simple an organism as a bacterium, the processes cannot con- 

 tinue uniformly, although the food supply, the water, and the temperature 

 may "remain the same" for a long period. For, as the cell grows in size, the 

 surface through which it absorbs and excretes enlarges more slowly than the 

 mass of protoplasm (see illustration, p. 345). The supply of food therefore 

 steadily diminishes for each unit of protoplasm, and excretion becomes slower 

 and slower. Sooner or later, then, every cell must stop growing. This is not 

 the only feature about living cells that sets a limit to indefinite growth, but it 

 suggests how a process may limit itself. 



Under conditions favorable to growth, a particular kind of cell — -a bac- 

 terium, for example — divides into two when it reaches a certain size. The 

 mother cell goes out of existence. It has not died, for the protoplasm of which 

 it consisted continues alive and active; but it no longer exists. 



Life Is a Pattern The external factors upon which living things depend 

 are not always uniform. But even where they are fairly constant (as deep in 

 the ocean or inside a warm-blooded host) each individual, each cell, has its 

 definite pattern of growth. In each species the individual grows and develops, 

 from stage to stage, in a relatively fixed or consistent pattern. Every stage of 

 life leads automatically to the next. And in most species this succession leads 

 to a "natural death". If we measure the intensity of metabolism by cell 

 division or by growth, we find a general slowing down. As the zygote starts 

 to grow, it doubles its weight several times in the first few days. A human 

 baby doubles its weight in the first six months after birth. Each year it 

 adds a smaller fraction of its weight, until growth becomes at last negligible. 



One cannot, by taking thought, add to his stature. Neither can one turn 

 back his developing, nor skip a stage, nor dally indefinitely along a pleasant 

 stretch. It is no wonder that men, reflecting upon life, have been impressed 

 with the idea of "fate" — which compels everything to happen in its appointed 

 time, everything to happen in its preordained spot in the great procession. 



What Causes Death? In spite of this picture of an irresistible and ir- 

 reversible march of events, life is anything but uniform. Individuals differ in 



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