240 General Discussion 



Drosophila. On the right you have the survival curves under the same 



K R 



conditions for the two reciprocal hybrids, - and — I thinj^ that is 



B K 



rather an instructive example of the effect of heterosis, of induced 

 hybrid vigour, on the life-span. The degree of increase — I did superim- 

 pose the pictures — is almost exactly comparable quantitatively to the 

 degree of decrease in life-span which was obtained in the rat experiments 



10 20 30 40 50 60 70 80 90 100 



Age in days 



Fig. 2. (Comfort) Drosophila subobscura. Survival curves of 



inbred lines B and K, and of the reciprocal hybrids between 



them (sexes combined.) 



Clarke, J. M. and Maynard Smith, J. (1955) .7. Genet., 53, 172. 



which Dr. McCay published. There was approximate doubling of life- 

 span and no change in the slope of eventual mortality; what has been 

 done is to push the whole thing over, with a much longer plateau. 



I wanted to show that as an indication of the extremely large effect 

 which can be produced by a genetic factor, in this case heterosis. Now 

 had we started with that particular hybrid cross, and inbred from it, I 

 have no doubt that over five or six more generations we should slowly 

 push the life table back again to the position of curves B and K, the equili- 

 brium position which we have when we have a line that has been 

 partially inbred for some time. That is rather an awful warning in 

 so far as these laboratory stocks are concerned. We can't, I know, go 



