294 Group Discussion 



may make a difference between the curve bulging up one way, or 

 bulging down the other. The largest possible dispersion of effects is 

 found here, which brings us to the idea of the 50 per cent survival 

 age point. To get a time scale which enables comparisons to be 

 made between different kinds of animals, on the other hand, you 

 need something which is not sensitive to that kind of change. This 

 suggests that you should use the peak of the curve of deaths, because 

 although the height of the peak is ^'e^y much correlated with antici- 

 pated deaths and so on, the actual movement of the peak is not 

 particularly sensitive. So while it is unprofitable to talk about an 

 ideal lifespan, it is quite practical and profitable to look at large 

 families of curves of death for the same species, and see what kind 

 of shape they tend to in general, so that you can get for the different 

 species a typical modal attained length of life, which you could use 

 as the time scale. 



Comfort: This is Bodenheimer's "physiological longevity" 

 (Bodenheimer, F. S. (1938). Problems of animal ecology. Oxford 

 University Press). 



Perks : I would like to plead that you should all take an interest in 

 the international actuarial notation. I think it is a very good thing 

 that all scientists should use the same notation if there is one which 

 is generally accepted. Dr. Benjamin has included some of it in his 

 paper and I think it probably could be extended to cover all your 

 needs. 



We have heard the phrases lifespan and ageing over and over 

 again but I do not think you will ever succeed in rigorously defining 

 them. I think they are best left as rather vague concepts, as we 

 know in general what we are talking about. 



I have been a little puzzled by the extent to which logs have been 

 taken of various observed figures. To me it only confuses the issues, 

 particularly when you have a graph on a logarithmic scale, although 

 I do understand that sometimes it is necessary to do that to com- 

 press the graph to reasonable dimensions. But there is no excuse for 

 taking logs in arithmetic merely to get rid of some of the variations, 

 and apparently to produce a correlation or regression which possibly 

 is not there if you do not take logs. What does the logarithm of a 

 residual mean? When you take a logarithm of the cephalization 

 index and then associate it with a logarithm of body weight, and 

 finish up with a logarithm of a lifespan, what does it all mean? 



There may be a very simple technique that might be useful for 

 those of you who study lifespans or mortality of animals in their 

 natural state. I understand that it is very difficult to get their ages, 

 but often I imagine you will find stationary populations in the natural 



