It wasn't conscious, but when I actually look and think what topics have I really spent a long time thinking about in biology, they tend to have been topics which are not easy to explain in Darwinian terms. And ageing, obviously, I mean, senescence says you get more likely to die as you get older. Well, why would you want to die at all? I mean, Darwinism would suggest you ought to last forever, and so on. So there is a problem, and always has been, for a Darwinist in explaining senescence. And I think that's why I got interested in ageing. And similarly, the other things I've been interested in, like sex and ritualised behaviour, have exactly the same kind of motivation, there's something going on that isn't easy to explain in Darwinian terms.

[Q] Did you talk to Medawar about ageing?

I don't think I did. I do remember Medawar - Medawar's inaugural lecture, when he came to University College, which was certainly before I was working on ageing, was about ageing. And he had some, I think, very perceptive things to say about ageing. I knew he was interested in the problem. I mean, he hired Alex Comfort, for example, who was also working on ageing. But I don't actually think that that's why I worked on the problem.

[Q] He produced the first, sort of, Darwinian account of it.

Yes, he did. No, no, Peter encouraged me a lot to work on ageing. But I think, curiously enough, again Helen comes out on top. I think I worked on ageing because the flies told me to, almost, you know. I mean, I was interested in the inbreeding phenomenon, and one of the things one couldn't help noticing, that the inbred animals didn't live very long. And that's when I started doing life table work. And then I got more and more chemical, it all got terribly technical towards the end. I mean we discovered a rather unexpected thing about flies, I mean this was much later, by the mid '60s, that fruit flies, or at least Drosophila subobscura, I don't think it's even been repeated on melanogaster, but most of the proteins in adult fruit flies are not replaced at all. None of the muscle protein, none of the mitochondrial protein is turned over at all in adult fruit flies. And by the time an adult fruit fly is a couple of months old, its mitochondria have become almost totally uncoupled. They will oxidise like crazy and not make any ATP, because they're made of proteins which have gradually come unwound and untangled and denatured and so on and they're just not working any more. And, in a sense, it was when we discovered that, which we didn't really finish until I came to Sussex, actually, that in a sense, I slightly lost interest in ageing. I mean, this seemed to me to be a sufficient explanation of why fruit flies age, why they become less and less capable of doing things as they get older; but it quite certainly isn't the explanation of why you and I age, because we do replace all our mitochondrial proteins. And I was forced to the conclusion that actually there's not one cause of ageing but many different causes of ageing, both independently in a single organism, many causes of ageing, and different causes in different organisms. And it sort of loses its universality somehow, you know. And anyway, it became too chemical for me, I wasn't going to start struggling with oxidative... phosphorylation in mitochondria, it's not my scene at all, so I got out.