152 Inside the Living Cell 



produce similar substances and apparently are constructed of similar 

 kinds of cells. The pancreas of different mammals produce insulins 

 and enzymes which are almost interchangeable. Yet, nothwithstand- 

 ing their close similarity, the life times of such cells within the dif- 

 ferent mammals vary enormously. This suggests that the determining 

 factor is not a characteristic of the individual cells, but something 

 which is determined by the organism as a whole. 



We are still ignorant of what this factor is which determines the 

 life span of animals. There are some suggestive facts. Frequently the 

 life span is proportional to the size of the animal. This may be con- 

 nected with the fact that the life span is proportional to the time taken 

 by the animal to reach maturity, i.e. to reach its full size with all its 

 organs functioning. It has even been shown that if the attainment of 

 maturity is artificially delayed, e.g. by keeping the immature animal 

 very short of food, the life span is thereby increased. Some fishes 

 which apparently continue to grow indefinitely also live for very 

 long periods. This suggests that ageing begins when growth ceases. 

 But why should the ageing process take so much longer with a large 

 animal like an elephant than with mice or rabbits? It would appear 

 that the ageing process is slower with a larger number of cells than 

 with a smaller collection. This may perhaps indicate that the cells 

 of an organ help each other and in a large organ there is a greater 

 possibility of the wear and tear, which must occur in some cells, being 

 made up by others. 



However, we must not suppose that increase of life span is neces- 

 sarily desirable. If there were no death, then there could not be any 

 new individuals either. Unless there were a continual stream of new 

 individuals, there could not be any evolution. Other things being 

 equal there will be a greater possibility of evolutionary changes if the 

 time from one generation to the next is as short as possible. The mini- 

 mum time is that required for the individual to come to maturity and 

 to produce and launch the next generation. From an evolutionary 

 point of view, any longer life-time is wasteful; but of course in order 

 that each generation shall be able to perform its duties, there has to 

 be some margin of viability and under favourable conditions, 

 animals will usually be able to live for a time after their useful repro- 

 ductive life has finished. From this, it would seem that the life span is 

 determined biologically by the time taken for the animal to reach 

 full maturity and produce offspring. The larger and more complex 

 the organism, the greater is the time required for this development. 

 Beyond this it may well be that there is an evolutionary advantage in 

 individuals not living too long and that this has led to the develop- 

 ment of factors in the organisms which result in ageing processes 



