THE UNIQUENESS OP THE INDIVIDUAL 



IX 



The postponement of the time of overt action of 'unfavour- 

 able** hereditary factors is not just a good idea which the 

 organism would be well advised to apply in practice; postpone- 

 ment may be enforced by the action of natural selection and 

 senescence may accordingly become a self-enhancing process. 

 Let me give you a real example in which this process appears 

 to be happening at the present time. 



Huntington ""s chorea is a grave and ultimately fatal nervous 

 disability distinguished by apparently compulsive and dis- 

 ordered movements akin to, and perhaps identifiable with, 

 'St Vitus'* Dance'. Its first full clinical description is in George 

 Huntington'*s own memoir of 1872, though the evidence I shall 

 appeal to comes largely from the fine treatise of Dr Julia Bell. 

 Huntington's chorea is a hereditary affliction of a rather special 

 sort. Its disabling and clinically important effects first become 

 manifest not in youth or old age but at an intermediate 

 period, its time of onset — later in men than in women — being 

 most commonly in the age-group 35-39. Its age of onset does 

 however vary, and I want you to assume (what is almost 

 certainly true, though it would be hard to collect the evidence 

 for it) that its age of onset, like the disease itself, is also 

 genetically determined. 



If differences in its age of onset are indeed genetically deter- 

 mined, then natural selection must so act as to postpone it: for 

 those in whom the age of onset is relatively late will, on the 

 average, have had a larger number of children than those 

 afflicted by it relatively early, and so will have propagated 

 more widely whatever hereditary factors are responsible for the 

 delay. But as the age of onset approaches the end of the 

 reproductive period, so the direct action of selection in post- 

 poning it will necessarily fade away. 



One may now ask why, if such a thing must happen, has it 



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