EXPERIMENTS ON LONGEVITY 



393 



observation, feed and care for them 

 properly, and observe the times at which 

 the individuals die, until the last one of 

 the thousand has ceased to live. 



The result of such an experiment is 

 shown in figure i. 



These curves bring out several important 

 points regarding the duration of life in 

 flies. The first is that there are differences 

 between individuals in respect of 

 longevity. Under identical conditions of 

 environment, housing, feeding, etc., some 

 individuals live longer than others, and 

 the distribution of these differences in 

 longevity is a regular and characteristic 

 one. In the second place, if we compare 

 the upper two lines of the diagram, which 

 depict the facts for normal, wild type 

 Drosophila, with the lower two lines, 

 which relate to the mutant form vestigial, 

 characterized by greatly reduced and dis- 

 torted wings, it is seen that these two 

 different races or strains of flies have, as 

 races, characteristically different average 

 durations of life and different distributions 

 of longevity under standard laboratory 

 conditions. Vestigial flies live, under 

 the same environmental conditions, only 

 about a third as long as normal wild type 

 flies, on the average. Finally the diagram 

 shows that female flies live longer than 

 males, on the average. 



The facts shown in figure i are descrip- 

 tive of the distribution of longevity in 

 populations of flies of two sorts, wild and 

 vestigial. But, like all descriptions of 

 natural phenomena, their primary effect 

 on the inquiring mind is that they de- 

 finitely formulate problems. What we 

 want to know is why it is that some 

 individual flies live longer than others, 

 and that some kinds of flies (normal wilds) 

 live longer than other kinds (vestigials). 

 When we say "why" in this connection, 

 what we really mean, of course, is that we 

 want to know what are the variables 



which are causally determinative of the 

 observed facts. Let us now see what can 

 be found out in this direction. 



INHERITANCE OF LIPE DURATION 



The first question which occurs to the 

 biologist when confronted with any such 

 problem is naturally as to what heredity 

 has to do with the results. Light upon 

 this query may be got in two different 

 ways. We may first try to determine 

 whether differences in longevity between 

 individuals of the same strain and in the 

 same population have a genetic basis. 



Average Duration of Life in Four Inbred Lines 





[soge rs. 

 \ndsys 



line 



Fig. i. Showing the Mean Duration of Life, in 

 Days, of Four Inbred Lines of Drosophila, All 

 Isolated Originally from the Same General 

 Population 



The solid bars show the duration of life in the line 

 at its first test, and the cross-hatched bars the dura- 

 tion of life in the same line tested about seven 

 months later. 



One way of doing this, which is standard 

 in genetic methodology, is to form inbred 

 lines through the process of successive 

 brother X sister matings, and then see 

 whether significant differences in lon- 

 gevity are permanently characteristic of 

 different inbred lines so produced. The 

 result of such experiments on duration of 

 life in Drosophila is to show that, in fact, 

 inbred lines or strains showing permanent 

 differences in longevity can be produced 

 by isolating and propagating in this way 

 individuals from a general mixed popula- 

 tion which, in current genetic terminol- 

 ogy, is not homozygous relative to the 



