Group Discussion 299 



senescence. The nature of this grouping probably has some moral 

 significance. 



A point that impressed me very much was the difficulty of evaluat- 

 ing the significance of data obtained from animals in captivity, which 

 are living in conditions of constant diet, no exercise, constant illumin- 

 ation, very little in the way of seasonal change, and so on, to which 

 even animals which have been selected for laboratory purposes are 

 not really 100 per cent adapted. After all, the period of adaptation 

 to laboratory life is comparatively short compared with the period of 

 evolution. I do not really know what can be done about that. What 

 is outstandingly important is that, wherever possible, a pathologist 

 should look at the animals when dead, and this should be done for 

 insects as much as for any other form of animal life. 



The concept of a biological time scale continues to interest us, 

 though it is obviously even vaguer than some of the other concepts, 

 which, as ]\Ir. Perks remarked, are better kept vague. However, this 

 particular one is of no use to us unless we can measure it. It is useless 

 to continue to use it without exact definition. 



Whatever the source, if we exclude accidents to which the individ- 

 ual concerned does not contribute, in most instances susceptibility 

 to mortality in a species or a strain does develop in a typical manner. 

 It is perfectly clear from what has been said that if one cause of 

 mortality is removed, for most of the animals for which we have had 

 data analysed in sufficient detail here, some other cause of mortality 

 would rapidly cause life to come to an end in the individuals which 

 have survived. The total gain of lifespan which would result from 

 eliminating one cause of death is not great. In other words, opera- 

 tionally we appear to be dealing with a unitary process. Whether 

 ageing is in fact unitary, however, cannot be determined from the 

 data we have been presented with so far. I am not at all clear to 

 what extent Maynard Smith's theory of synchronization of inde- 

 pendent lethal processes is a valid one, but it is, I am sure, a very 

 important matter indeed to have had raised, and one which necessi- 

 tates a good deal of further thought and investigation. 



The concept of a limit to the life of a tissue which is set in terms of 

 the energy conversion per unit mass per life cycle is a very attractive 

 one, but possibly dangerously attractive. If we accept it at its face 

 value, it presumably means that living matter commits accidents 

 at a rate which is proportional to the rate of energy conversion in it ; 

 this is quite a tenable view physically, and one which must surely be 

 open to investigation. We need more data, for example from the 

 study of hibernating animals, and from the use of metabolic poisons. 

 We ought to use metabolic poisons and radiation in attempts to 



