346 A 07-011 Franklin Shull 



A similar explanation is at hand for the "sex strains" described 

 by Punnett. The various series of generations in which this 

 investigator found constant differences in the proportion of male- 

 producers were probably reared on different food. Though the 

 dates of the experiments are not given, it seems almost certain, 

 from the author's account of them, that they were performed 

 at different times, hence different food must have been used. It 

 is thus possible that the constant differences which Punnett 

 noticed were due to constant differences in the nature and quantity 

 of the substances contained in the food cultures. It is conceiv- 

 able that "strains" may occur in the sense that families derived 

 from rotifers having very different histories may behave differently 

 with respect to the proportion of male-producers. Work is 

 needed to decide this point. But Punnett's explanation does not 

 apply to series of families derived from sister individuals, and re- 

 course to it is not necessary in other cases if the food cultures 

 are different. 



The influence of the character of the food cultures is again shown 

 in the experiments (V and VI) with old and new food. The roti- 

 fers fed from the old cultures include fewer male-producers than 

 do those given fresh food. As the protozoa in the two cultures 

 were apparently equal in all respects, the cause of the difference 

 in the proportion of male-producers must be sought in the liquid 

 portion of the culture. If it be supposed that the substances in 

 the cultures, which tend to reduce the number of male-producers, 

 accumulate with increasing age of the culture, the smaller pro- 

 portion of male-producers in families fed from old food is placed 

 in harmony with the other experiments. 



Such an accumulation of substances in old food cultures may 

 account for other phenomena. The experiments (XI and XII) 

 in breeding from the first and last members of the family may, 

 on this assumption, be brought into harmony with the general 

 conclusion. At the time when the first daughter in a family was 

 hatched, the food culture from which her mother was fed was three 

 to five days old; when the last daughter was hatched, the same 

 food culture was eight to eleven days old. The food culture 

 had grown older in the dishes with the rotifers, just as it had 



