THE PROBLEM 45 



"that the duration of life is directly linked with the degree of permeability 

 in that part of the living cell which places it in contact with the universe 

 about it, and that as the activities of life proceed the cell is being gradually 

 entombed by an inevitable decrease in the permeability of its protoplasm. 

 While decreasing permeability furnishes a possible explanation of the 

 more obvious symptoms of senility, it cannot be the only degeneration of 

 first rank. All protoplasmic functions must be involved. Underlying these 

 primary causes of senile degeneration there must be some general funda- 

 mental cause from which they spring. This fundamental cause may well 

 be the colloidal nature of protoplasm." 



Delage and Jennings have considered that death is 

 the result of differentiation. Jennings has put the matter 

 in this way 



"the continuity of life in the infusoria is in principle much like that in 

 ourselves, though with differences in details. As individuals, the infusoria 

 do not die, save by accident. Those that we now see under our microscopes 

 have been living ever since the beginnings of life; they come from division 

 of previously existing individuals. But in just the same sense, it is true 

 for ourselves that everyone that is alive now has been alive since the 

 beginning of life. This truth applies at least to our bodies that are alive 

 now; every cell of our bodies is a piece of one or more cells that existed 

 earlier, and thus our entire body can be traced in an unbroken chain as 

 far back into time as life goes. The difference is that in man and other 

 higher organisms there have been left all along the way great masses of 

 cells that did not continue to live. These masses that wore out and died 

 ,are what we call the bodies of the persons of earlier generations; but 

 our own bodies are not descended by cell division from these; they are the 

 continuation of cells that have kept on living and multiplying from the 

 earliest times, just as have the existing infusoria." 



Jennings' views regarding senescence in the protozoa 

 will be discussed in the next chapter. 



Unicellular organisms, as we have seen, do not nor- 

 mally experience natural death. In the higher organisms 

 there has been a progressive setting apart of cells and 

 tissues to perform particular vital functions with a con- 

 sequent loss of the ability to perform all vital functions 

 independently . As soon as any one of these cells or tissues 

 begins, for any accidental cause whatever, to fail to per- 



