THE UNIQUENESS OF THE INDIVIDUAL 



value of older age-groups will be apparent when I take the next 

 step in animating my test-tube model. The test-tubes are no 

 longer to be thought of as immortal; on the contrary, after a 

 certain age, as a result of some intrinsic shortcoming, they 

 suddenly fall to pieces. For the time being we shall assume that 

 they disintegrate without premonitory deterioration. What 

 will be the effect of this genetically provoked disaster upon the 

 well-being of the race of test-tubes? It must be my fault if the 

 answer does not appear to be a truism — that it depends upon 

 the age at which it happens. If disintegration should occur five 

 years after birth, its consequences would be virtually neglig- 

 ible, for under the regimen which we have envisaged less than 

 one in five hundred of the population is lucky enough to live so 

 long. Indeed, if we relied upon evidence derived solely from the 

 natural population of test-tubes, we should probably never be 

 quite certain that it really happened. We could make quite 

 certain, as we do with animals, only by domesticating our 

 test-tubes, shielding them from the hazards of everyday usage 

 by keeping them in a padded box as pets. 



If disintegration should occur one year after birth, an age 

 which is reached or exceeded by about one-quarter of the 

 population, the situation would be fairly grave but certainly 

 not disastrous; after all, by the time test-tubes have reached 

 the age of twelve months they have already made the greater 

 part of their contribution of offspring to the future population. 

 But with disintegration at only one month, the consequences 

 would obviously be quite catastrophic. 



This model shows, I hope, how it must be that the force of 

 natural selection weakens with increasing age — even in a 

 theoretically immortal population, provided only that it is 

 exposed to real hazards of mortality. If a genetical disaster that 

 amounts to breakage happens late enough in individual life, 

 its consequences may be completely unimportant. Even in such 

 a crude and unquaHfied form, this dispensation may have a 



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