62 MAN THE ANIMAL 



the children of short-lived, moderately long-lived, and very 

 long-lived parents? This we have done for sons as a class, with 

 the results shown in Figure 8. 



Plainly the results of these life tables for sons confirm the 

 conclusions derived from those for fathers that have just been 

 examined. As we pass upwards through the three broad classes 

 of paternal longevity the expectation of life of the sons at all 

 ages steadily rises. The expectation of life of the sons of short- 

 lived fathers is less than that of the sons of moderately longev- 

 ous fathers, and still less than that of the sons of extremely long- 

 lived fathers. Thus at age sixty the sons of very long-lived 

 fathers (eighty and over) have a further average expectation 

 of life nearly 40 per cent greater than that of sons whose fathers 

 died before age fifty. 



It seems unnecessary to present further evidence to demon- 

 strate the great significance of genetic factors in determining 

 individual differences in the length of human life. The inherited 

 biological constitution of each individual human being — his or 

 her genetically determined inherent viability — is beyond ques- 

 tion one of the major determiners of the probable length of that 

 persons's life. It is not, however, the sole or absolute determiner. 

 Obviously anyone can behave in such a way that his or her 

 genetic birthright in longevity is prevented from coming to 

 its full expression. Prematurely taking one's own life by the 

 suicide route is perhaps the most nearly perfect example. On the 

 other hand the general effect of public health and sanitary 

 measures is to create and promote such conditions of living as 

 will permit the greatest possible number of people to bring as 

 nearly as possible to complete realization and expression the in- 

 herent vitality with which they have been genetically endowed. 

 In the changing shapes of the Ladder of Life shown earlier 

 it has been seen how great the progress has been in this respect 

 for the earlier years, and how little for the later years of the 

 life span. This suggests, in the light of the evidence regarding 

 the inheritance factors in longevity, two conclusions that may 

 be of considerable significance. The first is that there exist broad 



