The Biology of Senescence 



combination of rapid growth with increased longevity analogous 

 to that of the rapid-growers described by Robertson and Ray 

 (1920). An example of the same effect in Drosophila is shown in 

 Fig. 33. The greater longevity of goldfinch-canary mules com- 

 pared with the parent species is apparently well known to 

 aviarists, and such techniques of crossing are of widespread 



Fig. 33. — Drosophila subobscura — hybrid vigour and longevity. Survival 



curves for the inbred lines B and K, and for the reciprocal hybrids between 



them (sexes combined) (from Clarke and Maynard Smith, 1955). 



agricultural and economic importance when applied to sheep 

 or to plants. 



The existence and magnitude of this effect should always be 

 borne in mind in the planning of experiments on the life-span 

 of animals drawn from closed laboratory stocks — such work can 

 produce very seriously misleading results if unrecognized 

 heterosis takes place. If an experiment in which the longevity 

 of generations is compared begins with hybrid progeny, marked 

 inbreeding depression can shorten the life-span of the succeed- 

 ing generations if genetical precautions are not taken. 



A fuller study of longevity in hybrids might provide useful 



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