AN UNSOLVED PROBLEM OF BIOLOGY 



real bearing on the origin of innate deterioration with increas- 

 ing age. There is a constant feeble pressure to introduce new 

 variants of hereditary factors into a natural population, for 

 ''mutation'', as it is called, is a recurrent process. Very often 

 such factors lower the fertility or viability of the organisms in 

 which they make their effects apparent; but it is arguable that 

 if only they make them apparent late enough, the force of 

 selection will be too attenuated to oppose their establishment 

 and spread. Such an argument may have a particular bearing 

 on, for example, the occurrence of spontaneous tumours and 

 the senile degenerative diseases in mice of which Dr Gorer has 

 made a special study, for these affections make themselves 

 apparent at ages which wild mice seldom, perhaps virtually 

 never reach. We only know of their existence through domesti- 

 cation; small wonder if they have no effect on the well-being of 

 mouse populations in the wild. Mice, of course, do already 

 show evidence of deterioration in the course of ageing, but 

 my reasoning does not presuppose it. It applies to '"poten- 

 tially immortal populations'* with only a quantitative loss of 

 cogency. 



It is a corollary of the foregoing argument that the post- 

 ponement of the time of overt action of a harmful hereditary 

 factor is equivalent to its elimination.^ Indeed, postponement 



^ As an example of what I mean by the time of 'overt action' of genes, I 

 should say that the earliest age of overt action of a 'coat colour' gene was 

 with the growth of a coat of hair in mice, which are born naked, or with 

 birth in animals like the guinea-pig, which are born with a pelt of hair. It 

 is not until hairs are both formed and exposed to outward inspection that 

 the various coat colours, as such, can influence the welfare of their pos- 

 sessors. But I agree with Dr Griineberg that one must be very cautious in 

 speaking of the time of action of genes — for one important reason among 

 several, because its influence on coat colour may be only one, and by no 

 means the most important one, of the manifold actions of what is only for 

 convenience of labelling described as a 'coat colour' gene. We have further- 

 more only the vaguest ideaof what we mean by speaking of a gene's 'acting' 

 at all. This particular difficulty can be overcome by accurate formulation: 



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