Group Discussion 287 



not, while in Drosophila I used only the imaginal lifespan and 

 ignored the whole of its previous larval career. 



I have not included among these examples the agricultural type 

 of lifespan, which Dr. Hartwig mentioned for his cattle and horses ; 

 that is yet another question. And I should add that in fish kept in 

 the laboratory we obtain a series of curves under different degrees 

 of environmental comfort which are very like those for mammals, 

 including man. In all cases the force of mortality rises with in- 

 creasing age. 



Muhlbock: The mouse has the advantage that there are inbred 

 strains available, as you know, and we use them in our work on 

 cancer research. These inbred strains come from brother-sister 

 matings, made for at least 20 generations, and all the strains I shall 

 refer to here have been through more than 100 inbred generations. 

 Survival curves for the females of the DBA and 020 strains show that 

 hybrids from the two strains live longer than the pure inbred lines. 

 An analysis of the DBA strain shows that the males die first and the 

 virgin females live longest; in between come the breeding females. 

 By breeders we mean females which are allowed to rear their young. 

 For special purposes in cancer research there is another group of 

 females which is described as force-bred. That means that the young 

 are discarded after birth, so that no lactation occurs. The lifespans 

 of the force-bred females are plotted from the 12th month, after 

 the fertility period has ended, so their shorter lifespan cannot be due 

 to accidents in pregnancy and there must be some influences which 

 affect lifespan in the second half of life when the fertility period has 

 ended. The CBA strain is one of the longest-lived strains we have; 

 some of the virgin females live to an age of 35 months. Here again 

 the virgin females have the longest survival. In this strain the males 

 are not so different, but the force-bred females, with rapid pregnan- 

 cies and without lactation, have a shorter survival. But that is not 

 the case in all the different strains we have. In the O20 strain the 

 differences between the various states are not so great as in the other 

 ones. There are therefore differences in these different strains but I 

 do not know what is the reason for them. 



Rockstein: In the life-tables of the male houseflies I was interested 

 particularly in the d^ values because from about the 10th to the 24th 

 day of life this represents a fairly large part of the population. We 

 are really at the peak of the mortality during this period of cohort 

 existence. In the females there is a grouping in the d^ values, so that 

 they reach a peak, then fall a little, and then reach another peak, and 

 so on. This illustrates the idea which the probit curve seems to sug- 

 gest, namely that the female is involved in a more complicated type 



