32 Discussion 



have emphasized this point. As for degenerative diseases, their role is 

 negligible ; their importance as a mortality factor in populations is really 

 a by-product of domestication (in animals) and of civilization (in man). 



Olbrich: The point is that you don't know the diseases. We know the 

 human diseases, we know what humans die of, we have the postmortems. 

 Your wild animals have various diseases that you don't know; you have 

 no postmortems. So your vital statistics are not the vital statistics we 

 deal with. Ageing must be something you can define and you, as a 

 scientist, must be able to tell me as a clinician, "These are the clinical 

 signs of ageing." And it must not be a disease; once it is a disease it is 

 no longer "ageing" but "accident of death". That is all right, the 

 mortality rate with respect to longevity, I accept that. 



Lorge: This point about variability being raised here is very interesting. 

 I have been trying to estimate whether ageing brings greater or lesser 

 variability. You look at a trait like vision and you find diminished 

 variability with age, but look at vigour and you find augmented varia- 

 bility. For an overall factor like vigour, subject to a heterogeneity of 

 causes, you get wider variability. The methodological issue is whether 

 or not multiple causes can be summarized simply into a gross trait 

 leading to the conclusion, "Here we have an increase in variability." 

 The probabilities are for an increase in variability, due to the fact of 

 two kinds of circumstance : one is disease, accident or special population, 

 and the other is the factors related to vigour. Now, those that are related 

 to vigour (with increasing age) tend to lead toward diminished variability. 

 In human populations we lack knowledge about the extremes of the 

 range, i.e. the people who are extraordinarily vigorous or who are extra- 

 ordinarily non- vigorous. They are the people we do not test, we cannot 

 get them to be examined. 



Olbrich: Even accepting what you say about vigour, more vigour or 

 less vigour is a clinical sign. 



Lorge: I agree. 



Olbrich: That is the reason for your great increase in coefficient of 

 variation. This signifies that the shape of Dr. Comfort's blood-pressure 

 curve * consists of two types of people, the pathological and the normal, 

 mixed together as one population. 



Lorge: There is actually a third population. This is very much like 

 the analysis of the life tables made by Arne Fischer years ago. He 

 suggested that difference in variabilities may be a function of two, three 

 or four sets of causes. I think his point raises the question of method- 

 ology very strongly. 



Bald: I agree with Dr. Olbrich that in studying the lifespans of different 

 animals we need to know their diseases. I studied recently arterio- 

 sclerosis in the dog. This research was undertaken because, following 

 the work of Anitchkow and co-workers, the rabbit has been the animal 

 generally used in studying arteriosclerosis and it has been suggested, 

 especially by American workers, that we should use some other animal, 

 for instance the dog, which is more like the human. In studying this con- 

 dition, I found that there is a very common parasitic disease of the dog, 



* Not submitted for publication. — Eds. 



