30 HELEN DEAN KING 



As the rats had been inbred brother and sister only, they were, 

 according to Fish's ('14) calculations, 79.687 per cent homozy- 

 gous at the time that the selection of breeding animals began 

 (seventh generation). Selection, if effective at all in changing 

 the sex ratio, should act in one or two generations, unless a con- 

 siderable number of factors were involved. In the latter case 

 selection might produce a gradual change in the sex ratio which 

 would reach its culmination only after a number of generations. 

 In each series, as table 4 and table 5 show, the sex ratio in the 

 inbred litters of the eighth generation was close to the sex ratio 

 that was the average for all of the litters produced in the eighth 

 to the twenty-fifth generations, and the sex ratios in the later 

 generations showed no greater deviation from the norm than did 

 those in the earlier generations, although they were somewhat 

 moreiftiiform. Selection thus produced its maximum effect at 

 once, and could not shift the centre of gravity of the variation in 

 the direction of the selection, as it did in the experiments which 

 Castle and Phillips ('14) made with piebald rats. It would 

 appear from these results that very few heritable factors concerned 

 in the production of the sex ratio, possibly not more than a single 

 pair, were acted upon by selection, and that, as Pearl ('17) has 

 stated: "selection acts only as a mechanical sorter of existing 

 diversities in the germ plasm and not as a cause of alteration in 

 it." 



As sister rats show such marked individual difference regarding 

 their sex tendencies, and as both nutritive (Slonaker and Card, 

 '18) and environmental conditions (King and Stotsenberg, '15) 

 seem to influence the sex ratio in the rat, it would seem that the 

 sex ratio may be modified by so many agencies that it would be 

 useless to attempt to determine the number or the nature of the 

 particular factors that were acted upon by selection in the present 

 case. The factors involved are evidently not of very great po- 

 tency, and their action is clearly shown only when a relatively 

 large number of animals are closely inbred under environmental 

 and nutritive conditions that are as uniform as it is possible to 

 make them. Whatever their nature, or in whatever manner they 

 may be inherited, I believe that these factors act on the metabo- 



