582 FRANK A. HAYS 
Mean head length is the sum of the averages of the three read 
ings for each individual in a litter divided by the number of 
individuals in the litter. Mean iliac extremes represents the 
sum of the average of the three measurements divided by the 
number in litter. By using the above formula for mean dimen- 
sion, the calculation was made for each litter for each of the 
nineteen periods of observation. Graphs presented on the 
measurement data are made up in exactly the same way as has 
been described for making the graphs for weight, comparing 
only litters of the same number of individuals. A grand average 
graph is likewise made up regardless of litter size. The graphs 
based upon identical litter size are considered reliable for pur- 
poses of comparing the offspring in the different service groups, 
but the grand average graph is subject to considerable error. 
DATA AND RESULTS 
1. Growth in weight of young as related to frequency of copulation 
of sure 
For studying the offspring with a view of determining if there 
is any relation between the number of services made by the males 
and the rate of growth, there appears to be no better measure 
than body weight. Body weight measures the animals as a 
whole and should thus reveal any inherent weakness that retards 
their growth. - 
Below are presented charts 1 to 12 taken from the composite 
weight records of the young of all three males. Each chart rep- 
resents a single litter number at birth, all litters of one size, in 
each of the five service groups being grouped together and the 
' same grouping being followed for all litter numbers as described 
on page 580.' Each service group is represented by a different 
line, as is disclosed by the legend on the charts. The number 
of litters represented by each curve is given in each case. Chart 
13 shows the weighed grand average for all five service groups. 
It was obtained by adding together the individual mean of each 
litter and dividing by the total number of litters at each obser- 
vation period. We have already shown that such a chart com- 
paring directly the growth ratio of the different service groups 
