Discussion 283 



about 50 days, and which does not seem to be reversible, can properly 

 be regarded as a process of ageing in its own right. Therefore I am 

 rather unhappy about thinking of exposure to 30 • 5° simply as the 

 application of a stress. 



Rotblat: Our own results could be explained by either single or 

 multiple processes, but like Sacher I think your results do not 

 necessarily contradict the single theory. You assume that high 

 temperature produces only ageing processes. But it may cause 

 something else ; it may cause trauma or some other process which is 

 not normally present, and consequently it may not be just an ageing 

 process. 



Griineberg: The question of whether we are dealing with single or 

 multiple processes of ageing could probably be tackled by investigat- 

 ing the effects of individual genes on the ageing process. In the ex- 

 periments you described, considerable differences were found 

 between different inbred strains. These, of course, differ in a multi- 

 tude of genes and in practice it is impossible to sort out the effects of 

 individual genes following a cross between two inbred strains, as I 

 have repeatedly found in my mouse crosses. It would probably be a 

 better plan to start with single-gene differences. We are about to do 

 that in mice, to see whether genes without obvious pathological 

 effects in some way affect the longevity of the animal. In the mouse 

 this will take about three years, whereas if you are so inclined, you 

 could probably produce significant results in Drosophila by Christ- 

 mas. 



Maynard Smith: I certainly could produce results by Christmas. 

 R. Pearl (1928. The Rate of Living. University of London Press) 

 found a long time ago that the gene vestigial in Drosophila melano- 

 gaster halves the expectation of life. I have no doubt that a number 

 of other genes would alter the lifespan. But I am not a good enough 

 insect pathologist, and to learn anything from such an experiment 

 one should analyse the causes of death. 



Griineberg : The gene vestigial is not a good gene to use because it is 

 itself obviously pathological: because of their reduced wings the 

 vestigial flies tend to get stuck in the food. One should use genes 

 that have no obvious pathological effect. 



Maynard Smith: Suppose that you find a particular gene, which is 

 not obviously pathological, but which reduces or extends the life. 

 It would not tell you much unless you could then show that animals 

 with the gene did not die of a specific cause that the others are dying 

 of, or vice versa. 



Griineberg: That is exactly what I mean. Once you have shown 

 that a gene affects the lifespan, whether it has an effect one way or 



