Group Discussion 297 



looking at natural populations, too. The problem now becomes to 

 try and find some general law which is applicable in spite of the 

 fantastic variety of conditions in the field. I think one would never 

 hope to find any common causes of death associated with the actual 

 time of death in nature. Every environment differs in its hazards 

 frona every other one. The point is, can we find anything in the 

 properties of the animals which does obey some sort of general law 

 under all these varied circumstances? In other words, what is it that 

 makes an animal increasingly likely to die regardless of what 

 actually kills it in the end? We might divide the problem into both 

 multiple and single processes. The final causes of death would be the 

 multiple processes, and there we have very little hope of introducing 

 much unity; but by concentrating on susceptibility there may be 

 some hope of finding a common process which can equally well be 

 studied in those animals which live to a great age in the laboratory, 

 and those which die at a much younger age in nature. Exactly how 

 one goes about this I do not know, and that is the point at which my 

 work is hung up. I have to account for very violent changes in the 

 probability of survival at different times of the population cycle, and 

 exactly where does one go to look in the organism for something 

 which may be an index of this change in properties? That seems to 

 me to be very similar to the problems with which one is faced in 

 trying to account for the increasing probability of death with age. 

 Animals in the field very seldom live to an age at which you can say 

 they are senescent. Nevertheless, it is a fact that even at a much 

 younger age than they die at in captivity, some species periodically 

 show this very great increase in probability of dying (see Green, R. G. 

 and Evans, C. A. (1940). J. Wildlife Mgmt, 4, 220, 267, 347). The 

 question arises of whether or not we should regard these as problems 

 of senescence and ageing, or as much more analogous to the high 

 probability of human beings dying young, or whether age is irrelevant 

 and some index of physiological condition is the only thing worth 

 trying to find in any species. 



Tanner: There does not seem to me to be any connexion between 

 the situation in non-domesticated animals and the situation in the 

 human. All the other mammals and birds would have died early in 

 terms of human growth. The human was the only animal which 

 seemed to be surviving long enough to experience senescent processes. 

 Cellular ageing might paradoxically haA^e been closer to the situation 

 in man, than is the situation in the passerine, for example. After 

 what Dr. Chitty has said I feel like withdrawing this comment, 

 because if in other species the probability of their dying in the field 

 increases with age, this is the fundamental thing. It comes back to 



