THE CIRCLE OF LIFE 5 



be a mature period of a great many years, as in the various 

 trees or in the larger animals. In man full maturity lasts 

 about fifteen or twenty years; that is, from thirty years of 

 age to forty-five or fifty. These figures are somewhat 

 misleading, inasmuch as some of our qualities may not 

 reach maturity by thirty, and others begin to wane before 

 we reach thirty. 



7. Decline and Old Age. We may think that old age 

 is just a matter of time. A little thought, however, will 

 convince you that this is a very small part of the story. 

 A grasshopper or a mouse "grow old" while a child or 

 a colt of the same age is still young. It is not primarily 

 time but something within which makes old age, and the 

 decline in power that comes with it. In some way the 

 aging plant or animal cannot continue to adjust itself to 

 the conditions about it. This adjustability to conditions 

 we find to be the real essence of life. For a while 

 the individual meets fully and profits by the surrounding 

 conditions. It receives stimuli, responds to them, and 

 becomes better because it does this. It learns, so to speak, 

 how, through its experiences, to adjust itself the better. 

 Thus it grows, matures, thrives. After a time, perhaps it 

 loses some of its plasticity or softness, its tissues harden, 

 its habits harden, and it no longer meets all its changing 

 needs so well. 



Then, too, it may be that all organisms, and all the cells 

 in the organisms, make poisons in the very act of living, 

 which make it harder to live. For example, if a number 

 of people are shut up in a room, we know that they give 

 off poisons which check and presently stop life. The same 

 is true of many germs that multiply in our bodies and 

 elsewhere. They quite frequently produce substances that 

 make it harder for themselves to live. If this is true of the 

 cells in our bodies, we can see how by a slow wearing out or 

 a slow poisoning each might do its work less and less well, 

 and thus a decline of the whole organism would follow. 



