198 BIOLOGYj^OF DEATH 



duration of life is a normally inherited character. I do 

 not wish at this time to go into any discussion of the 

 details of the Mendehan mechanism for this character, 

 in the first place, because it is too complicated and tech- 

 nical a matter for discussion here* and, in the second 

 place, because the investigations are far from being com- 

 pleted yet. I wish here and now merely to present the 

 demonstration of the broad general fact that duration 

 of life is inherited in a normal Mendelian manner in 

 these fly populations- The first evidence that this was 

 the case came from some work of Dr. E. E. Hyde with 

 Drosophila some years ago. The numbers involved in 

 his experiment, however, were much smaller than those 

 of the present experiments, and the preliminary demon- 

 stration of the existence of pure strains relative to dura- 

 tion of life in Drosophila was not undertaken by him. 

 Hyde's results and those here presented are entirely 

 in accord. 



With the evidence which has now been presented re- 

 garding the inheritance of life in man and in Drosophila 

 we may let that phase of the subject rest. The evidence 

 is conclusive of the broad fact, beyond any question I 

 think, coming as it does from such widely different types 

 of life, and arrived at by such totally different methods as 

 the statistical, on the one hand, and the experimental, on 

 the other. We may safely conclude that the primary agent 

 concerned in the winding up of the vital clock, and by 

 the winding determining primarily and fundamentally 

 how long it shall run, is heredity. The best insurance 

 of longevity is beyond question a careful selection of 

 one's parents and grandparents. 



* Full technical details and all the numerical data regarding these and 

 other Drosophila experiments referred to in this book will shortly be 

 published elsewhere. 



