XIX 



Ageing and Death 



H 



We have followed as far as we can the extraordinary developments of 

 the life of cells. We must now turn to another question and ask why, 

 if their organization is so perfect and well adapted to their functions, 

 should they not continue to live, failing accidents, at least as long as 

 they are provided with the necessary nutriments and with a suitable 

 environment? 



One could perhaps draw a distinction between unicellular organ- 

 isms and multicellular organisms. It has often been suggested that the 

 former are effectively immortal, since they undergo repeated sub- 

 division and, given enough food material, they need never die. How- 

 ever, the distinction between unicellular and multicellular organisms 

 in this respect is probably more apparent than real. A colony of cells 

 will go on increasing its numbers by cell division until it has used up 

 the food materials in its particular environment. No colony has the 

 whole world to expand into, if for no other reason than that, sooner or 

 later, it will come into competition with other organisms. When the 

 available food has been used up, the majority of the cells must die, 

 and new cells can be formed only so far as fresh food becomes avail- 

 able. It often happens also that a colony of cells destroys its environ- 

 ment by producing waste products, e.g. acids, in which it cannot grow. 

 Many unicellular organisms developed ways of escaping from this 

 situation, e.g. by producing spores, which may be able to establish a 

 new colony elsewhere. But in this case the colony is in a similar situa- 

 tion as that of multicellular organisms, which die after forming new 

 individuals from special cells like the sperm and egg. The colony of 

 single cells behaves very similarly when it has exhausted its habitat. 

 It dies, but a few cells or spores may escape and found a new colony 

 elsewhere. 



