Data for the Problem of Evolution in Man. 



293 



actual correlation between constitutional strengths to resist death 

 would be given, at any rate approximately, by the values determined 

 for other characters in a memoir on the Law of Ancestral Heredity,* 

 we have clearly a method of to some extent ascertaining the proportion 

 of the selective and non-selective death-rates in man. In the sequel 

 it will be shown that from the age of 20 to the end of life our tables give 

 a correlation between the duration of life of father and son of about 

 0*12 to 0-14, and between brother and brother of about 0'26. Accord- 

 ing to the Law of Ancestral Heredity we should expect these quantities 

 to be about 0*3 and 0*4. Hence we conclude that the amounts of 

 correlated material in the two cases are 40 to 50 per cent, and 65 per 

 cent. But if be the number of cases in which the death-rate is 

 selective for N individuals, y 2 N will be the number of cases for which 

 it is selective when we take pairs of individuals. In other words the . 

 selective death-rate in the first casef is about 63 to 70 per cent, and in 

 the second about 80 per cent, of the total death-rate. Without laying 

 great stress on the actual numbers just stated, we think that they are 

 sufficiently close to demonstrate that a substantial selective death- 

 rate actually exists at work on mankind, and that with like environ- 

 ment it may amount to as much as four times the non-selective death- 

 rate.:}: In other words, having demonstrated that duration of life is 

 really inherited, we have thereby demonstrated that natural selection 

 is very sensibly effective among mankind. The natural selection we 

 are here dealing with is not in the first place, of course, a result of any 

 struggle of individual with individual, but of individual with environ- 

 ment and with the defects of personal physique. 



4. In order to show the biological importance of investigating the 

 inheritance of duration of life, we have cited the results obtained for 

 correlation between the ages at death of father and son and brother 

 and brother. But the method by which these results were obtained 

 requires further discussion. We have already seen the need to exclude 

 deaths under 20 years, but even then we have not got in the case 

 of father and son two like groups of material. The father has been 

 more severely selected than the son. He has lived to become a father, 

 and he is strong enough to be the father of a son who lives to be 



* ' Koy. Soc. Proc.,' vol. 62, pp. 397 and 400. 



t This selective death-rate from the data for father and son must be interpreted 

 in the sense indicated above. The drop from 80 to 65, say, per cent, is in itself a 

 measure of the change of environment of the two generations. 



t The correlation on which this determination is based might be illusory, if 

 families were reared under very individual environments ; the correlation in 

 duration of life of brothers, for example, might then be a result of their individual 

 family environment. But the environment when we take comparatively homo- 

 geneous classes like the Peerage or Landed G-entry must be very similar, and we 

 think this source of error, suggested to us by Professor Weldon, while very real 

 has been sufficiently provided against. 



