THE UNIQUENESS OF THE INDIVIDUAL 



'immortar age distribution. It is by no means difficult to 

 imagine a genetic endowment which can favour young animals 

 only at the expense of their elders; or rather, at their own ex- 

 pense when they themselves grow old. A gene or combination 

 of genes that promotes this state of affairs will under certain 

 numerically definable conditions spread through a population 

 simply because the younger animals it favours have, as a 

 group, a relatively large contribution to make to the ancestry 

 of the future population. It is far otherwise with a genetic 

 endowment which favours older animals at the expense of 

 young. Reflection will show that the gene or genes concerned 

 cannot plead for a retrospective judgement in their favour; for 

 before the animals which bear these genes give outward *'pheno- 

 typic"* evidence of the fact, they are on equal terms with those 

 that do not. The greater part of the ancestry of the future 

 population will thus have been credited indifferently to both 

 types, because a gene qualifies for the preferential action of 

 natural selection only when, to put it crudely, it manifestly 

 works. This does not imply that a late-acting gene which 

 confers selective advantage cannot spread through the 

 population. It can indeed do so; but very much more slowly 

 than a gene which gives evidence of itself earlier on. The 

 later the time in life at which it appears, the slower will be 

 its rate of spread; and the rate in the end becomes vanishingly 

 small. 



The consequence of any decline in the fertility of older 

 animals is cumulative. Once it has happened, a new set of 

 events may be put in train. Consider the fate of genetic factors 

 that make themselves manifest in animals that bear them, not 

 at birth nor in the first few days of life but at some time later 

 on. Quite a number of such genes are known, and what is said 

 of them applies equally to genes which have an expression, 

 but a variable form of expression, throughout the whole span 

 of life. It may be shown that if the time of action or rate of 



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