12 THE CAVENDISH LECTURE 



strated that on the average the longer lived leave sensibly more 

 offspring. In the next place it was proved that both physical 

 and mental characters were definitely inherited. Then, if the 

 third point could be demonstrated — namely, that there was 

 a selective death-rate in man — there could be no flaw in 

 the argument that Darwinism applied to man ; from a 

 mere plausible belief it would become a law of human life. 

 How was this selective death-rate to be determined ? How was 

 o-eneral physical fitness to be measured ? It occurred to me that 

 taken all round physical fitness was properly measured by length 

 of life. If length of life in the case of human beings was a per- 

 fectly arbitrary quantity, then the shafts of death were random 

 shots. Death did not select his victims. In other words the 

 question of the selective death-rate in man was the question of 

 the random character of length of life. Now if length of life 

 depends on physical constitution and power to resist disease, then 

 on the Darwinian theory these characters would be inherited, and 

 length of life would on the average be related in parent and off- 

 spring. Briefly, if there be no selective death-rate there will be 

 no relation between lengths of lives of parent and offspring, or 

 generally between members of the same stock ; if all deaths be 

 selective, then the relation between durations of life will be the 

 same as that of any physical character in parent and child. As a 

 matter of fact neither one nor other position is true ; not all deaths 

 are selective, but we can determine what proportion are. In the 

 case of individuals living under the same environment over 75 per 

 cent, of the deaths were thus shown to be selective. 



That demonstration that some 75 per cent, of deaths depend 

 upon hereditary constitution seems to me conclusive, but it has 

 not seemed so to a distinguished member of the Local Govern- 

 ment Board, although my conclusions have been verified by a 

 distinguished biologist, Prof. Ploetz, working on German, not 

 English material. Dr. Newsholme has recently published a 

 Government report on infantile mortality, and, working in con- 

 junction with Mr. G. U. Yule has discovered that when the 

 infantile death-rate is high in a given locality, then the death-rate 

 of children of a greater age during the same period is also high. 

 This result has apparently been used to demonstrate that a high 

 infantile death-rate does not mean a low child mortality, and 

 indirectly by one of its authors and a whole army of party news- 

 papers to preach that the Darwinian doctrine has no application 



