The Influence of Genetic Constitution 



Drosophila segregates were identified in the F x by subsidiary, 

 anatomical characters known to be associated with the desired 

 lines. In Drosophila subobscura of the structurally homozygous 

 Kiissnacht strain, which had been in culture for about three 

 years, and had reached an equilibrium life-span considerably 

 shorter than that of wild-caught flies, breeding for 8 generations 

 over 1 year exclusively from eggs laid after the thirtieth day of 

 parental life produced no significant alteration in mean imaginal 

 longevity (Comfort, 1953) (Fig. 32). 



The inheritance of long life in man is presumably bound up 

 with the inheritance of 'general health' (Pearson and Elderton, 

 1913; Pearl, 1927) an element which is not more susceptible to 

 analysis than 'vigour', though it has been partially described in 

 terms of response to stress (Selye, 1946). Robertson and Ray 

 (1920) found that in a population of mice the relatively long- 

 lived individuals formed a stable sub-group, displaying the 

 least variation and the highest resistance to disturbing factors. 

 In such a group the growth-rate tends to be a measure of 

 'general health', and rapid rather than retarded growth corre- 

 lates with longevity. In other studies on groups of animals living 

 under standardized conditions, rate of growth and length of 

 life have been found to vary independently (Sherman and 

 Campbell, 1935). The relation between growth-rate and vigour 

 in a mixed population requires to be distinguished from the 

 effect of growth retardation by dietary means in a homogeneous 

 population, when the retarded growers live longer. As McCay 

 (1952) points out, much early work on the relation between 

 growth-rate and longevity was vitiated by this confusion in 

 experimental planning. 



4-1-2 PARENTAL AGE 



The age of the mother is known in certain cases to modify the 

 longevity of her offspring. This influence apparently include a 

 wide range of dissimilar effects, some strictly 'genetic', and 

 others operating at various stages in the process of embryo- 

 genesis, or, in mammals, on into lactation. Certain of these 

 effects appear only in the F l5 while others, like the factor 

 described by Lansing in rotifers, which leads to a decreas- 

 ing life-span in successive generations of clones propagated 



125 



