Rate of ageing in Drosophila suhohscura 271 



evolved molar teeth which grow throughout life, whereas 

 large herbivores have not. Similarly, the age of onset of 

 cancers in species with different life expectancies (say in mice 

 and men) is roughly proportional to those expectancies, and 

 this proportionality seems more likely to be a consequence of 

 synchronizing selection than of a direct physiological con- 

 nexion between ageing generally and cancer. 



It follows that a decision between a single and a multiple 

 theory in any species is impossible without experimental 



Table I 



Mean survival times in days of adult flies at various 



temperatures 



interference with the process. The grafting of organs from 

 young individuals into old and vice versa, or between individ- 

 uals with different genetically determined rates of ageing, is 

 perhaps the most promising experimental approach (Jones and 

 Krohn, 1959; Medawar, 1957). Some experiments w411 now be 

 described on ageing in Drosophila suhohscura, using a different 

 approach, but leading to the conclusion that the ageing pro- 

 cess is a multiple one (Maynard Smith, 1957, 1958). 



It has been known for a long time that in poikilothermous 

 animals the expectation of life decreases with increasing 

 temperature. It was the purpose of the investigation now to 



