General Discussion 



239 



eight generations of Drosophila subobscura. They were bred in each 

 generation from the eggs hiid by flies after tlie thirtieth day of iniaginal 

 hfe. The objeet of the experiment was to try to find tlie same thing as Dr. 

 Lansing found in rotifers, and to see whether tlie life-span of the off- 

 spring of old parents decreased cumulatively. It didn't do so in this 

 experiment; there is no significant trend in those figures over eight genera- 

 tions occupying about a year. But even more interesting is the stability 

 of the life-span. In each case we were breeding from the longest-lived 



Fig. 1. (Comfort). Survival curves oi Drosophila subobscura strain 



K, bred in each generation from eggs laid by parents after the 



30th day of imaginal life. 



members of the cohort. You'll see that in each case the thirty-day mark 

 comes down to about the 75 per cent mortality point, we had only about 

 a quarter of the flies left on each occasion, and only quite a small pro- 

 portion of those were fertile. But by inbreeding long-lived flies, in a strain 

 which had been in laboratory culture for some three years, and which 

 was genetically fairly stable, we were not able to effect any appreciable 

 increase at all in the imaginal life-span. I've got permission from my 

 colleagues Miss J. Clark and Dr. Maynard-Smith (J. Genet., 53, 172) 

 to show an experiment which they did on the same strain of Hies. Fig. 2 

 shows the survival curves for (dotted line on the left) the same 

 Kiissnacht strain which was used in my experiments, and (solid line on 

 the left) a strain B which was an inbred line from another kind of 



