290 Group Discussion 



the probability of being hit according to the number of times the 

 object has been hit before, and if you proceed further and increase 

 the speed of the shooting, the mathematics develop in the direction 

 of the Makeham curve and certain modifications of it. 



Maynard Smith: I do not believe for one moment that the shape 

 of these curves has anything to do with the organism that has been 

 studied. I think it has something to do with the environment in 

 which it was studied. 



Perks: I have thought about this problem for a long time and I 

 believe the shape has something to do with time. 



Tanner: You would expect a different-shaped curve in a less long- 

 lived organism? 



Perks: No, I think it is the cumulative factor in life; injuries and 

 so on are additive, or cumulative in almost a geometrical sense, and 

 you must expect to have exponentials coming into the form of the 

 mortality rates. 



Tanner: Does this make the change, then, from the ganmia func- 

 tion into the Makeham type of curve? You think that the reception 

 state changes according to the number of shots impinging? 



Perks: Yes. 



Holt: In the models for fitting the depreciation of motor cars 

 death is assumed to result from either single big accidents, or an 

 accumulation of small ones, and a complex death curve follows; it 

 seems to have some possible application to animal mortality. 



Rotblat: I am concerned to know what lifespan is because we have 

 to express some of our findings in terms of lifespan. For example, 

 people often speak about the effect of radiation in causing a shorten- 

 ing of the span of life and they put down figures of the percentage 

 of fife- shortening per rontgen. But everyone I have asked what he 

 meant by the term lifespan gave a different answer. I was hoping 

 that perhaps in this meeting we might come to some agreement on 

 that. I was impressed by your remarks on the first day. Dr. Benja- 

 min, when you brought in what you called the senescence peak. 

 This peak seems to me to be the quantity to put down as the *' normal 

 lifespan". I was particularly interested to see data presented by 

 various people which showed that although the shapes of the 

 mortality curves may differ enormously, nevertheless for a given 

 species they all reach nearly the same end-point. If we could draw 

 the ideal curve, i.e. if we could eliminate all deaths due to accidents, 

 infections, etc., then the "normal lifespan" would be obtained im- 

 mediately from the senescence peak. But in practice this will be 

 very difficult, because most mortality curves do not approximate to 

 this ideal. We are, therefore, still left with the question of what we 



