At the age of twelve years, the human body is at its most vigorous. It has yet to reach its full size and strength, and its owner his or her (1) f________ intelligence; but at this age the likelihood of death is (2) l________. Earlier, we were infants and young children, and consequently (3) m________ vulnerable; later, we shall undergo a progressive (4) l________ of our vigour and resistance which, though imperceptible at first, will finally become so steep (5) t________ we can live no longer, however well we look after ourselves, and however (6) w________ society, and our doctors, (7) l________ after us. This decline in vigour with the passing of time is called ageing. It is one of the (8) m________ unpleasant discoveries which we all make that we must decline in this way, that if we escape wars, accidents and disease we shall eventually ‘(9) d________ of old age’, and that this happens at a rate which differs little (10) f________ person to person, so that there are heavy odds in favour of our (11) d________ between the ages of sixty-five and eighty. Some of us will die sooner, a few will live longer—on into a ninth or tenth decade. But the chances are against it, and there is a virtual limit on how (12) l________ we can hope to remain (13) a________, however lucky and robust we are.

Normal people tend (14) t________ forget this process unless and until they are reminded (15) o________ it. We are so familiar (16) w________ the fact that man ages, that people have for years assumed that the process of losing vigour with (17) t________, of becoming more likely to die the (18) o________ we get, was something self-evident, like the cooling of a (19) h________ kettle or the wearing-out of a (20) p________ of shoes. They have also assumed that all animals, and probably other organisms such as trees, or even the universe itself, must in the nature of things ‘wear (21) o________’. Most animals we commonly observe do in fact age as we (22) d________, if given the chance to live long enough; and mechanical systems like a wound watch, or the sun, do in fact run (23) o________ of energy in accordance (24) w________ the second law of thermodynamics (whether the whole universe does so is a moot point at present). But these are not analogous (25) t________ what happens when man ages. A run-down watch is still a watch and can be rewound. An old (26) w________, by contrast, becomes (27) s________ worn and unreliable that it eventually is not worth mending. But a watch could never repair itself—it does not consist of living parts, only of metal, which wears away by friction. We could, at one time, (28) r________ourselves—well enough, at least, to overcome all but the most instantly fatal (29) i________and accidents. Between twelve and eighty years we gradually (30) l________ this power; an illness which at 12 would knock us over, at 80 can knock us out, and into our grave. If we could stay as vigorous as we are at twelve, it would take about 700 years for half of us to die, and another 700 for the survivors to be reduced by half again.