596 FRANK A. HAYS 
formity in weight at birth should be expected. The thirty-day 
period is the weaning time for all of the litters studied in this 
experiment. We note from the table that the coefficient of 
variation falls below what it was at birth in all service groups. 
- Here again there is no evidence of an increased percentage of 
‘weak’ offspring in advanced service groups, for if such were the 
case we should expect the coefficient to increase when the ani- 
mals were thrown into competition for nutrition during the first 
thirty days of postnatal life, and even one inferior individual 
' would alter the coefficient for the litter. At the ninety-day 
period there is again a decrease in the coefficient of variation in 
all service groups, except the 15th- and 20th-service groups. 
The large size of the probable error here indicates that the 
15th-and -20th-service groups cannot safely be assumed to be 
exceptions. 
Taking up a comparison of the coefficients for the different 
service groups, there appears to be slightly less variability in 
the offspring as the number of services increases, but this de- 
crease is not universal. Since the probable error is rather large, 
this difference is no way significant. As has been previously 
stated, there is also a slight reduction in the number in the 
litters in the same direction. Our data show us further that 
there is less variability in the smaller than in the larger litters. 
This fact affords us an explanation for the slight reduction in the 
coefficient of variation as the number of services increases. 
In table 3 a further fact seems apparent that occasional 
genetically weak offspring do not occur in any one of the service 
group more frequently than in any other service group. The 
table also shows us that for the first ninety days of postnatal 
growth there is a tendency for individuals of the same litter to 
approach nearer to a mean weight than was the case either at 
birth or at thirty days of age. Fetal nutrition is thus more vari- 
able than either the nutrition furnished by the mother during the 
first thirty days after birth or the ordinary food supply fur- 
nished from thirty days to ninety days. 
