Longevity of Man and his Tissues 
seems therefore worth doing. The other important demo- 
graphic factor is whether or not our reproductive period is to be 
increased correspondingly. It almost certainly would be in 
men, and it might be in women. It has been suggested that the 
suppression of ovulation by Dr. Pincus’s pills might have this 
effect in any case through ovum-sparing and the cutting down 
of losses from atresia. This might in turn have some unwelcome 
genetic effects—but it might well be that the disincentives to 
reproduction after the present limiting age will be strong enough 
to limit its demographic significance. 
The most important single change in our world, where life- 
span is concerned, is that in privileged countries our children 
grow up and reach old age and our wives no longer die in 
childbirth. Men have always known the probable limit of their 
lives. We now know more accurately than ever before when we 
are likely to die. The most important future change depends 
on the progress of our understanding of fundamental age 
processes. If the present trend of medicine continues without 
such progress, all that will happen is that the commonest age 
of dying will shift from being nearer 75 to being nearer 85, and 
the commonest causes may change so that we die of conditions 
which are not now so common, today’s most frequent killers 
having been removed to uncover the next layer of the onion. 
If this is all, not many more than the present 2 in 100 born will 
reach go, and not many more than the present 1 in 1,000 will 
reach 100. Those who do will still be the progeny of long-lived 
stocks, and owe more to their parents’ genes than to medical 
science. 
If, on the other hand, fundamental interference does become 
possible, so that we can modify the rate of ageing itself, the 
picture will be different. It might prove possible, first of all, to 
lengthen the period of adult vigour without increasing the final 
lifespan. This would produce a nearly square survival curve 
with its limit short of the century, and a situation like that in 
Aldous Huxley’s Brave New World, where people remained 
apparently young until great ages and then died suddenly at 
approximately the usual time. This seems biologically the least 
8* 227 
