EXPERIMENTS ON LONGEVITY 



395 



genetic factors which may be presumed to 

 determine longevity. Examples of this 

 are shown in figure z. 



This diagram demonstrates conclusively 

 that there exist in a general population of 

 Drosophila large differences in longevity 

 which can be made permanent by breeding. 

 As a matter of fact some of these lines are 

 still carried as stocks in the laboratory, 

 and have preserved substantially un- 

 changed the average duration of life shown 

 in figure i for eight years, without further 

 selection, or brother X sister mating. 



Another kind of evidence demonstrating 

 the inheritance of longevity in Drosophila, 

 under constant and defined environmental 

 conditions, can be obtained by crossing a 

 long-lived strain with a short-lived strain, 

 and analyzing the results in successive 

 generations of progeny in the usual 

 Mendelian manner. Such crosses were 

 first made between normal wild type flies 

 and the mutant vestigial. Figure i shows 

 that these two kinds of flies have markedly 

 different average durations of life and 

 different life curves. 



The chief, broad results of such a cross- 

 ing experiment are shown in figure 3 . 



When flies carrying the mutation ves- 

 tigial are crossed with normal, wild type 

 flies, the wing mutant character vestigial 

 behaves as a simple Mendelian recessive, 

 without sex-linkage, since the gene for 

 vestigial is located in the second chromo- 

 some. The first generation flies from such 

 a cross are all of the normal wild type, 

 with normal wings. When these first 

 generation flies are mated with each 

 other, they produce a second generation 

 in which individuals bearing normal 

 wings and individuals bearing vestigial 

 wings occur in the theoretical proportion 

 of three of the former to one of the latter. 

 The actually realized proportions approxi- 

 mate to this theoretical ratio. 



In respect of duration of life the facts 



are these. The vestigial strain used as 

 parent in the cross had an average duration 

 of life of approximately 14 days. The 

 wild type strain used as the other parent 

 had an average longevity of 44 days. 

 In the first generation flies, which are all 

 of normal wild type morphologically, the 

 mean duration of life was approximately 

 52. days. When these first generation flies 

 were bred with each other the wild type 

 flies of the second generation had a mean 

 duration of life of 46 days, while the 

 recessive vestigial flies of this second gen- 

 eration had a mean duration of life of 

 only 13 days. There thus appears a clean 

 segregation in respect of average lon- 

 gevity exactly paralleling that in the 

 morphology of the wings. The forms 

 segregated in the second hybrid generation 

 have the same average duration of life as 

 the corresponding original parent forms, 

 to within one or two days. 



A great many experiments of this type 

 have been carried out in my laboratory, 

 involving a number of mutations other 

 than vestigial. What they all show 

 clearly is that duration of life behaves 

 like a segregating character in the Men- 

 delian sense. How are these facts to be 

 interpreted? 



The most reasonable interpretation, con- 

 sonant with all the facts, seems to be to 

 assume that duration of life, considered of 

 itself, is not a biologically separate char- 

 acteristic of the organism, but instead is 

 simply the expression of the total func- 

 tional-structural organization or pattern 

 of the individual. It is apparently this 

 organization or pattern which is in- 

 herited, and not duration of life as such. 

 Duration of life is only one of many 

 objective manifestations of the thing 

 which is inherited. Put in another way, 

 what this view of the matter says is that 

 if the duration of an individual's life is an 

 implicit function of the individual's or- 



