OLD AGE AND NATURAL DEATH 



expression of such genes is itself genetically modifiable, then, 

 if the gene confers selective advantage, its time of action or of 

 optimal expression will be brought forward towards youth, as 

 it spreads through the population. If, by contrast, the gene is 

 'disadvantageous**, then its time of action or threshold of un- 

 favourable expression will be pushed onwards in life while it is 

 being eliminated from the population. The former process may 

 be called a precession of favourable gene effects; the latter, a 

 recession of unfavourable effects. Neither process can come into 

 operation unless the fertility of the population declines with 

 age, so that the reproductive value of its members falls; and 

 the latter process, the recession of unfavourable gene effects, 

 will be modified by the fact that the later an ""unfavourable"* 

 gene comes into operation, the slower will be the process of its 

 removal from the population. (At some critical late age, per- 

 haps, an unfavourable gene is eliminated so slowly that natural 

 selection cannot challenge its reintroduction into the popula- 

 tion in the process of gene mutation.) The precession of 

 'favourable'' gene effects will in its turn be modified by the fact 

 that reproduction cannot start at birth, and nature has found 

 in higher animals only the most indirect substitutes (maternal 

 care, and the blunderbuss of huge fecunditv) for the theoretic- 

 ally desirable state of affairs in which an animal is born 

 mature. Because of the hazards to which baby animals are 

 exposed (and this is just as true of human beings) the repro- 

 ductive value of the individuals always rises to a maximum 

 before eventually it falls; and it is at the epoch of this maxi- 

 mum, therefore, that the 'precession"* of favourable gene effects 

 will automatically come to halt. It is not surprising, then, to 

 find that in human beings the 'force of mortality"* is lowest just 

 when the reproductive value would in the members of a prim- 

 itive society be highest — in the neighbourhood of the four- 

 teenth or fifteenth years of life. ^ Nor is it surprising to find that 

 ^ A correlation pointed out by R. A. Fisher (see note 1, p. 37). 



39 



