Ageing and Death 151 



However, this analogy is rather superficial. The multicellular 

 animal is usually mobile and can get away from its waste products. 

 It does not have to die because it has either used up the nutriments 

 around it, or because it has fouled its environment. Are we to sup- 

 pose that it dies because it has fouled its 'internal environment', i.e. 

 that waste products gradually accumulate inside the organism, which 

 clog the efficient working of the different organs and in the end cause 

 death? 



It is certain that in many organisms ageing processes do occur. 

 Even with animals kept in good surroundings with adequate food and 

 protected from other predatory organisms the organism as a whole 

 gets less efficient. The bodily processes slow down, the muscles be- 

 come less effective, the bones often become brittle, many tissues 

 undergo degenerative changes — e.g. tendons become less elastic, and 

 growth processes, e.g. growth of hair and healing of wounds, slow 

 down, with the result that the repair of damages caused by wear and 

 tear becomes less effective. Finally, some vital organ ceases to func- 

 tion and the organism as a whole ceases to live. 



It has been very difficult to discover why organisms age in this way 

 and what determines the average length of life. In the natural state 

 ageing processes are rarely encountered because death usually occurs 

 owing to some mischance while the animal is still in its prime. Yet 

 there is no doubt that animals which are carefully maintained, live 

 for various characteristic average periods. It has been suggested, as 

 I mentioned above, that organs become clogged by waste products, 

 or by substances of a poisonous nature which they are unable to 

 get rid of. This may be so, although it has not been clearly demon- 

 strated. In a more general sense it can be supposed that cells will only 

 function efficiently for a limited period. Membranes may break 

 down or become clogged and synthetic mechanisms may begin to 

 work incorrectly. Accidental damage may occur in the cell which it 

 is unable to repair. 



On the other hand, it seems to be true that tissues which are capable 

 of regenerating themselves by cell division will continue to function, 

 possibly indefinitely. Cell cultures continue to grow (by cell division) 

 in artificial media for many generations, long after the animal from 

 which the cells were obtained has died. Sea urchins live indefinitely, 

 because all their cells undergo continuous replacement. 



This would seem to indicate that the cell has a limited life time and 

 will die unless it is able to regenerate itself by cell division. This does 

 not explain why animals have such very different life spans. Different 

 mammals are extraordinarily similar in their general construction. 

 They have very similar organs which function in similar ways and 



