Rate of Ageing in Drosophila subobscura 281 



ageing processes of females at 20°, or is itself an age processing 

 capable independently of causing the death of females. 



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Clarke, J. M., and Maynard Smith, J. (1955). J. Genet., 53, 172. 



Griffiths, J. T., and Tauber, O. E. (1942). Physiol. ZooL, 15, 196. 



Jones, E. C, and Krohx, P. L. (1959). Nature {Lond.), 183, 1155. 



IVIaynard Smith, J. (1957). J. exp. Biol., 34, 85. 



Maynard Smith, J. (1958). J. exp. Biol., 35, 832. 



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DISCUSSION 



Danielli: There may be an alternative explanation for your 

 experiments to the one you suggest, namely that the causes of 

 ageing are multiple. In the study of cell division it is now common 

 practice to synchronize cells by giving them a cycle of temperature 

 changes. The logic behind this is that the synchronization is due to 

 the breaking of a cycle or to interference with some phase of the 

 cycle of metabolic activity, so that when the constraint due to 

 temperature change is removed, all the cells take up a new cycle at 

 the same point. In your animals the variance decreased in some of 

 the experiments, which would suggest that some measure of syn- 

 chronization was occurring. You may fail to get the additive effect 

 one expects in ageing, not because the cause of death is different at 

 different temperatures, but because you break the initial ageing 

 cycle by moving from one temperature to another and then later 

 the animals begin somewhat closer to the origin of a cycle than they 

 would have done had you kept them constantly at one temperature. 

 This would mean that there is possibly only one cause of death, 

 although they are behaving as if there were two causes. 



Maynard Smith: I would accept that as a very possible explanation 

 of the reduction in variance of the population exposed to 30 • 5° inter- 

 mittently, compared to that exposed continuously. We shall have 

 to repeat the experiment which showed this striking reduction in 

 variance to see whether it was just one of those things that happen 

 once, or whether it will happen every time. But I would not accept 

 your suggestion as an alternative to the existence of the two processes 

 of ageing. After eight days at 30 • 5° one knows that, although all the 

 animals are alive, they are all actually "half dead". They are all 



