106 INBREEDING AND OUTBREEDING 



attributed partly to selective eUmination of the weaker 

 rats cannot be determined. We are inclined to agree with 

 Miss King in giving greater weight to the first factor, 

 though for a reason which she does not mention. The 

 general success of Miss King^s whole investigation we 

 believe to be due largely to the fact that the experiments 

 were started with stock rats which already must have 

 been very closely inbred and therefore in an approxi- 

 mately homozygous condition. 



From the seventh generation on, selection was made 

 on the new-bom young with general vigor as the basis, 

 but the two lines were selected differently. In line A only 

 litters having an excess of males were selected to serve 

 as the progenitors of the succeeding generation, while in 

 line B the reverse was the case. The general result was 

 to show that the normal sex ratio in this species, 105 males 

 to 100 females, can be changed. At the end of nineteen 

 generations of selection, line A had produced litters hav- 

 ing a sex ratio of 122.3 males to 100 females, and line B 

 had produced litters having a sex ratio of 81.8 males to 

 100 females. From these facts there is no doubt but that 

 lines having an hereditary tendency to produce different 

 sex ratios can be isolated, but there is no evidence what- 

 ever in favor of the theory of Diising proposed in 1883 to 

 the effect that inbreeding by lessening the vitality of the 

 mother increases the percentage of male young. The 

 change in the sex ratio was made in two generations. 

 After that the effect of selection ceased. Such a result 

 not only militates against attributing the changed ratios 

 to inbreeding itself, but indicates that a relatively small 

 number of Mendelian factors are involved in the control. 



The effect of continuous inbreeding on body weight is 



