UNIT SEVEN — REVIEW • WHY CANNOT PLANTS 



AND ANIMALS LIVE FOREVER? 



With our own strong desire to live, it is natural for us to seek ways of 

 lengthening individual life, as well as of enriching it. And with the use of 

 modern knowledge we have indeed stretched the average duration of human 

 lives in this country by more than ten years since the early part of the present 

 century. It is likely that we shall succeed in reducing the death-rates at the 

 younger ages still further. But under the most favorable conditions there is 

 still a limit to the length of individual life, and we need not search for physical 

 immortality. Is, then, the life of the individual self- limiting? 



The more we study the activities and the processes of plants and animals, 

 the clearer it becomes that it could not be otherwise. Although cells of dif- 

 ferent tissues or of different species vary greatly in size, each cell reaches a 

 limit of growth. This limit seems necessary because the interchange of ma- 

 terials between the protoplasm and its environment is limited to the ratio of 

 the surface of the cell to the mass. 



There is a further limit in the fact that as the individual grows, the parts 

 become more and more specialized. Now living depends upon a close co- 

 ordination of all the parts. But handicaps or incapacities increase as minor 

 injuries accumulate in specialized structures which cannot regenerate or be 

 repaired. Finally, growing older involves accumulating wastes; lime, silica, 

 and other inert matter are deposited and so reduce the metabolic activities 

 in proportion to the total protoplasm. 



A different set of conditions limits both individual and total life. In the 

 whole world there is only so much carbon, only so much phosphorus, only so 

 much nitrogen — a limited amount of each of the elements essential to living 

 protoplasm. These materials are so distributed that only a fraction of the 

 total present is available for living things — in the waters and in the soils near 

 the surface of the earth. And even then they are present in proportions that 

 permit only a fraction of the accessible materials to be used by plants and 

 animals. There is, in fact, a surplus of one or another of these elements almost 

 anywhere, but that does not make up for those that happen to be deficient. 

 Now, li all the available materials essential to living things should at any time, 

 and in a particular region, become embodied in living plants and animals, 

 there would be the largest possible amount of protoplasm — and of "life". 

 But then, that condition could last for but a moment; for all the organisms 

 would immediately proceed to starve, or they would begin to destroy one 

 another. In either case, that "maximum" amount of life could not continue. 



Living depends upon a continuous flow of materials. Each individual is a 

 center of interchange of materials: this is a basic relationship between an 

 organism and its environment. Some species are related to one another 



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