270 J. Maynard Smith 



ageing processes were arrested, and, unless there is contin- 

 uous tooth growth, would ultimately lead to death. Similarly, 

 in so far as cancer is the result of cumulative environmental 

 insult, it is partially independent of other ageing processes, 

 though it would be rash to assume that it is wholly so. But it 

 is always possible that apparently unrelated symptoms of 

 ageing may be due to a single cause, just as apparently un- 

 related abnormalities of development may be the pleiotropic 

 effects of a single gene. A "single" theory would postulate 

 that all or most of the symptoms of ageing are the consequence 

 of a single process (or of a single series of processes), either at 

 a cellular or organism level. 



There is one observation which at first sight appears to 

 support such a single theory. In a given species, the deter- 

 ioration of different organ systems proceeds at roughly the 

 same rate; if this were not so, individuals dying of "old 

 age" would always die of the same immediate cause. This 

 synchrony might suggest a high degree of physiological inter- 

 dependence, with some one particular process acting as a 

 timekeeper. But the synchrony can be explained in another 

 way. Suppose that ageing in mammals is in fact multiple in 

 character. Then if in any species one ageing process, say the 

 deterioration of the central nervous system, proceeded at a 

 much higher rate than did other ageing processes, there 

 would be strong natural selection tending to slow down the 

 rate of ageing in this system, if necessary at the expense of 

 accelerating other ageing processes. In other words, natural 

 selection will tend to synchronize different ageing processes, 

 even if these are physiologically independent of one another. 

 The example of tooth wear already mentioned demonstrates 

 that selection can in fact act in this way. The volume of tooth 

 worn away in unit time is proportional to the volume of food 

 eaten, which in turn is roughly proportional to the surface 

 area of the animal. Consequently, the expectation of life of 

 the teeth of small mammals is less than that of large mam- 

 mals. However, many small herbivores (rodents) have 



