282 Discussion 



halfway through a programme towards death, and if left at that 

 high temperature most of them would be dead in another eight days. 

 If it was the same programme which is responsible for death at 

 20°, then one would expect that flies exposed for eight days to 30 • 5° 

 and then kept at 20° until they died would behave as if they were 

 halfway through the programme, and not, as actually happens, 

 back at the beginning. 



Danielli: I was actually suggesting that the abrupt change of 

 temperature swung the animals back to the beginning of the cycle in 

 each instance. This would mean that you could take the animals 

 halfway through their expectation of life at 30 • 5° and then the actual 

 change in temperature swings them back to the beginning of their 

 cycle again, or somewhere closer to it. 



Maynard Smith: If that were true, one could presumably make 

 them almost immortal. You are suggesting that flies kept at a 

 constant temperature die of physiological boredom. 



Sacher: I do not know whether your evidence clearly establishes 

 the hypothesis of multiple as against a single cause of ageing. We 

 had a diff'erent experimental situation which leads to results similar 

 to yours. Fruit-flies were given daily doses of X-rays throughout 

 their lives from emergence onwards. Under these circumstances 

 flies that received about 1 • 5 to 3 kilorontgens per day throughout 

 life lived more than 30 per cent longer than their controls and at the 

 same time manifested a markedly decreased variance. Subsequently 

 I discovered that W. P. Davey (1917, 1919. J. exp. Zool, 22, 573; 28, 

 447) had also done this with flour-beetles. My interpretation is that 

 X-irradiation is a stress, and that a moderate degree of stress invokes 

 adaptive responses that are not invoked in the animal's natural 

 environment. This leads me to ask whether you could do an experi- 

 ment in which temperature shocks are given daily or at frequent 

 intervals? 



Maynard Smith: I shall have to do such an experiment. Whether 

 you do or do not accept my conclusion that we have a multiple 

 process here hinges largely on whether you accept my argument that 

 what ultimately kills them at the high temperature is properly re- 

 garded as a process of ageing. If you just starve a population of 

 Drosophila they will all die in about three days, and you will get a 

 survival curve with an increasing force of mortality, looking just 

 like a life-table. But I do not think any of us regard this as a 

 proper process of senescence because it is fully reversible ; if you give 

 the flies food after two days' starvation they recover completely. I 

 am arguing that the process at 30-5°, which takes as long as 18 days 

 to reach completion in a fly whose normal life-expectation is only 



