STUDIES ON THE DURATION OF LIFE 197 



the cross of the long-lived and short-lived types, a group 

 of animals appears having almost identically the same 

 form of mortality curve as that of one of the original 

 parents in the cross. The mean duration of life of this 

 long-lived second generation group is 43.3 A days, 

 while that of the original long-lived stock was 44.2 -4 

 days. The short-lived F 2 segregates, shown at the bottom 

 of the diagram, give a mortality curve essentially like 

 that of the original short-lived parent strain. The two 

 curves wind in and about each other, the F 2 flies showing 

 a more rapid descent in the first half of the curve and a 

 slower descent in the latter half. In general, however, 

 the two are very clearly of the same form. The aver- 

 age duration of life of these short-lived second generation 

 segregates is 14.6 .6 days. This, it will be recalled, 

 is almost identically the same average duration of life 

 as the original parent Type IV gave, which was 14.1 

 .2 days. 



It may occur to one to wonder how it is possible to 

 pick out the long-lived and short-lived segregates in the 

 second generation. This is done by virtue of the corre- 

 lation of the duration of life of these flies with certain 

 external bodily characters, particularly the form of 

 the wings, so that this arrangement of the material can 

 be made with perfect ease and certainty. 



These results show in a clear manner that duration of 

 life, in Drosophila at least, is inherited essentially in 

 accordance with Mendelian laws, thus fitting in with a 

 wide range of other physical characters of the animal 

 which have been thoroughly studied particularly by 

 Morgan and his students. Such results as these just 

 shown constitute the best kind of proof of the essential 

 point which we are examining namely, the fact that 



