594 FRANK A. HAYS 
Chart 13 represents the grand average growth of all litters in 
the five service groups as explained on page 582. Each graph 
thus represents the individual mean for the combined litters in 
each service group. ‘These composite service group graphs bear 
out the general deductions that we have madefrom a study of the 
graphs taken one by one comparing litters of a given number 
with each other in the five service groups. There is one out- 
standing objection to the use of such graphs as are shown on 
chart 18. There is a perceptible negative correlation between 
number of services of the sire and the number of offspring in 
litters resulting (Lloyd-Jones and Hays, p.492). In other words, 
heavy service does reduce the size of litters, especially in the two 
most advanced service groups used here. Consequently the 
greater supply of nutrients furnished by the mother in utero as 
well as the greater supply of milk available after birth will 
enable the advanced service litters to outstrip the other litters 
during the periods of observation in this experiment. This con- 
dition would hold if all litters were equally fit genetically; and 
we have no evidence that any class of offspring is rendered [ess 
fit by heavy service of their sire. 
To recapitulate, certain errors have been introduced into the 
growth studies in body weight, chief among which are environ- 
mental factors, the age and weight of the dam and the weight of 
the sire. These errors have been partially corrected, and the 
conclusion seems justified that there is no evidence in this data 
to show that the amount of sexual service that the male has been 
required to perform in any way affects the rate of growth of his 
offspring in body weight for the first ninety days of postnatal 
life. 
2. Litter coefficient of variability 
The coefficients of variability in table 3 presented below were 
obtained in the following manner: The coefficient of variability 
for each litter in each of the five service groups was determined 
at birth, at thirty days, and at ninety days by the formula: 
Standard deviation of each litter. 
Mean of the litter. 
