Longevity of Man and his Tissues 
—silicosis, for instance—in having a cumulative factor super- 
imposed on an age-dependent increase. Psychosomatic stress, 
and particularly, it seems, the type of stress we find in prosperous 
modern urban communities which are anxious and bored 
(what I would tentatively call the Ulcer Belt Syndrome), 
might conceivably have a more fundamental effect on the 
rate of ageing, if the Pavlovians are to be believed—or it 
may be simply one more factor of environmental attack. But 
there are two cases where an adverse change in lifespan 
may be more fundamental: chronic over-nutrition, and 
radiation. 
Of these two possible causes of change in the rate of natural 
ageing, I think the second is the more convincing. Over-eating 
by adults certainly shortens life, but there is probably no 
fundamental difference actuarially between this type of shorten- 
ing and the shortening induced by malnutrition: they both 
represent increased environmental attack. The case for a 
fundamental life-shortening hinges rather on the more rapid 
development of modern children, and the consequent abbrevia- 
tion of childhood. The age of puberty has, it is said, advanced 
by about four months per decade over the past century. The 
argument is that as the lives of rodents can be prolonged 
nutritionally by keeping them immature, rapid growth is likely 
to reduce human life by the amount that adolescence is 
advanced, if not by more. I do not myself find this at all 
convincing. Genetic sexual precocity, though it is, of course, 
not strictly analogous to dietetic acceleration, does not notice- 
ably shorten life, and there is strong ground for thinking that 
the present trend represents not an unusual shortening of 
childhood, but a return to normal after an epoch of unusual 
retardation. I will not pursue this argument here, beyond 
saying that the present age of menarche in privileged countries 
is about what it was in Roman law!, and in the days of the 
Abbé Brantéme. 
One specific threat to our natural lifespan is the increasing 
use of ionizing radiation in medicine and industry. ‘This is 
clearly the major source of extra radiation exposure today, but 
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