AN UNSOLVED PROBLEM OF BIOLOGY 



have been slow to realize that the biologically important con- 

 sequences of this secular increase in average longevity began 

 to be apparent three-quarters of a century ago and are now on 

 the threshold of completion. About seventy-five years ago, the 

 mean expectation of life at birth in England and Wales began 

 to exceed, as it now greatly exceeds, the age beyond which 

 child-bearing virtually ceases. Women have had nearly all their 

 children by the time they are forty-five, but may novv expect, 

 on the average, to live some quarter of a century longer. The 

 fertility of men lasts beyond that of women and ends less 

 sharply, but, roughly speaking, three-quarters of the male 

 population is still alive at an age at which it can be credited 

 with 99 per cent, of its children. The principal causes of death 

 have changed accordingly. Fifty years ago the major killing 

 diseases were pneumonia and tuberculosis, both of infective 

 origin; to-day they are cancer and what is compendiously called 

 cardiovascular disease. Susceptibility to both cancer and the 

 cardiovascular diseases is in some degree influenced by heredity, 

 and should therefore be subject to those forces, of ''natural 

 selection**, that discriminate between the better and the genet- 

 ically less well endowed. (To speak of '"discrimination"* is, of 

 course, to put the matter in too literary a way; let us say that 

 people with different hereditary endowments do not have 

 children in strict proportion to their numbers; some of them 

 take more than their numerically fair share of the ancestry of 

 future generations.) But cancer and the cardiovascular diseases 

 are affections of middle and later life. Most people will already 

 have had their children before the onset of these diseases can 

 influence their candidature for selection. In the post-repro- 

 ductive period of life, the direct influence of natural selection 

 has been reduced to zero^ and the principal causes of death 

 to-day lie just beyond its grasp. 



^ The word 'direct' is important. Grandparents, though no longer fertile, 

 may yet promote (or impede) the welfare of their grandchildren, and so 



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