EFFECTS OF INBEEEDIl^G Al^D CROSSBREEDING. 17 



CA, in which inbred females are mated in various ways, and Experi- 

 ments CC, Cl, and AC, in which the females are crossbred. A priori, 

 one might expect that the inherent vigor of the yomig would count 

 for something, and the slight improvement in Experiments CO and 

 CA can be interpreted in this way. The superiority of Cl to CC, 

 however, does not support this view. These experiments differ only 

 in that the young of Cl are to some extent inbred. The superiority 

 of CA over CO might be interpreted as due to a direct influence of the 

 vigor of the sire, but the similar superiority of AC over CC is opposed. 

 All of these minor differences may, of course, be due to chance. 



Turning to the other experiments, we find that C2 is intermediate 

 between the group with inbred dams and the group with crossbred 

 dams. This is to be expected, since in C2 the dams as well as the 

 young are inbred, though not as much as the dams in A, CO, and CA. 

 Theoretically, as will be brought out later, C2 should be just halfway 

 between the inbreds and Experiment CC. 



Experiments CL and CG have averages close to the other cross- 

 breeding experiments. The superiority of CL over CG seems to be 

 due merely to chance. Recalling that the mated animals m CG were 

 exceptionally vigorous when weaned, while those in CL were only 

 two-thirds as heavy and usually unthrifty in appearance, it must be 

 concluded that the females completely recover from setbacks early 

 in life as far as ability to bear young successfully is concerned. It 

 may also be concluded that the superiority of Experiments AC, CC, 

 and Cl over the inbreds is not due in the shghtest to unconscious 

 selection of relatively more thrifty animals in makmg the matings. 



In this discussion we have been considering the indices, in which 

 there is correction for the effect of size of litter. In Figure 20 Experi- 

 ments Co, CC, Cl, B, and the inbreds are compared, taking each size 

 of litter separately. These experiments were in progress practically 

 at the same time, so that it is not necessary to make a correction for 

 seasonal effects. The inferiority of the inbred females (inbreds, CO) 

 is clearly brought out. It also appears that this inferiority is most 

 marked in the larger litters. Large litters are at only a slight dis- 

 advantage as compared with small ones in the case of crossbred dams, 

 but have a markedly higher death rate than small Utters in the case 

 of inbred dams. 



Summing up, the fate of the young at birth depends primarily on 

 the breeding of the dam, as far as it is genetic at all. Crossbred 

 females mated in any way are able to produce a distinctly larger 

 percentage of living young than females of the best of the inbred 

 families which entered into their composition. The inferiority of 

 inbred females is greatest in large litters. About half of the superi- 

 ority of the crossbreds is lost in the progeny of females from, the first 

 6448— 22— BuU. 1121 ^3 



