244 H. J. MULLER 



DISCUSSION 



MOLE: I would like to raise one or two questions about what I thought Prof. Muller 

 accejDted as demonstrated facts. For example, the longevity of cells in tissue culture. This 

 may be true of some kinds of cells but recent work in transplantation from mammal to 

 mammal seems to suggest that there is a limit to the life of some cells. He also said that 

 life-span was fixed by natural selection. I wonder whether this is a wholly unambiguous 

 statement. The life-span observed in nature is not necessarily the same thing as the 

 potential life-span which an organism wUl show living in some kind of fixed condition. 

 This seems to me to make the life-span of any kind of organism under laboratory 

 conditions not necessarily the same thing as the life-span in — let's call it — nature. Can 

 that be true of man who has only got himself free from some of the selective forces in the 

 last few tens of generations? 



mullek: Well, with regard to the first question I am quite prepared to admit that there 

 may eventually be an agemg of the somatic cells in culture, takmg place long after the 

 ageing of the individual as a whole, which was conditioned by other tissues. That would 

 not prove, however, that the ageing in culture was of genetic origin. It might be like the 

 clonal ageing which you referred to. 



In regard to the other point there are several considerations. For one thing it must be 

 remembered that man and most higher organisms — organisms in general — have had to 

 face quite varying conditions and become adapted to them. Thus they are quite versatile. 

 And while man under primitive conditions doesn't usually live as long as he does among 

 ourselves there are circumstances under which he does. It's very curious, for instance, 

 that we should find the statement in the Bible written so many years ago that threescore 

 years and ten is the natural age of man. We haven't gone very much beyond that yet, 

 really, for most of us. I think studies among the most primitive people show, too, that al- 

 though the great majority die offearly there are some that do live that long and these people 

 generally have a function. I think Haldane and some others too *nade a considerable 

 mistake in assuming that selection stops after reproduction stops because the reproduc- 

 tive function — in its more general aspect of furthering the survival of the species — goes 

 far bej'ond the more direct reproductive activities of having offspring. For the grand- 

 parents have a great deal to do especially amongst primitive peoples, in helping to ensure 

 survival of the grandchildren, and so I think there has been a use in their old age. 



Moreover, one must remember that even though a certam contingency isn't too 

 frequent a species wiU nevertheless adapt to it — and here I give Haldane credit, for he 

 once pointed out that species might be found adapted to some conditions, such as extreme 

 cold, which came perhaps only once m ten generations. Even that is enough to give a 

 natural selection pressure which will cause that species to become adapted to that degree 

 of cold. We don't generally recognize that as being part of the normal life of the organism 

 but it comes often enough for the organism to have become adapted to it. In the case of 

 life-span there have obviously been considerable genetic changes in our rather recent 

 evolutionary history, as sho^vn by the considerably longer life-span of man than of the 

 apes, the longer hfe-span of the apes, in turn, than that of the monkeys, and so on. So it is 

 very flexible, genetically, and I thhik has been fixed bj^ natural selection Hke anythmg else. 

 ALEXANDER: I wondcr whether the loss of a chromosome which as you said, is going to be 

 so serious in the case of germ cells and eggs is going to be so disastrous for a somatic cell? 

 For example, tumour cells which seem to be capable of looking after themselves very 

 well often show serious chromosome abnormahties. 



muller: Well, in our experiments with Drosophila the evidence showed that the cells can 

 stand having a chromosome missing provided the homologous member is there. If they 



