EFFECTS OF INBREEDING AND CROSSBREEDING. 17 
CA, in which inbred females are mated in various ways, and Experi- 
ments CC, C1, and AC, in which the females are crossbred. A priori, 
one might expect that the inherent. vigor of the young 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 in 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 slightest to unconscious 
selection of relatively more thrifty animals in making 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 C0, 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 litters 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—Bull. 1121——3 
