SEXUAL ACTIVITY OF MALE RABBITS 611 
heavy service group are more inferior as far as ability to in- 
crease in body weight is concerned than the offspring in the light 
service groups. 
The service group coefticient of variability does not reveal 
that any inferiority of entire litters is brought about by heavy 
sexual service of males. This coefficient does show that the 
largest coefficient of the first ninety days of postnatal life is 
found just at the close of the suckling period at thirty days. 
The coefficient further shows that the variability in weight of 
the general population is much greater than within the litters. 
Body measurements furnish us with further material for the 
study of the offspring in the different service groups. These data 
do not reveal any new facts to indicate any greater inferiority of 
offspring in any one of the five service groups. Here again the 
same modifying factors have been in operation that have af- 
fected the body-weight data, and a correction, if possible, for 
these we think would show that the offspring in all five of the 
service groups are identical in body dimensions. 
Concerning the question of rate of mortality in progeny from 
light and heavy service, we have no evidence that there is a 
higher death rate in the advanced service groups over that ob- 
served in the light service groups. 
A direct relation apparently exists between the amount of 
sexual service of males and the percentage of females that they 
will sire. The ratio of males to females is highest in the Ist- 
service group and progressively decreases up to the 20th-service 
group. There is a possibility that heavy service exerts a selec- 
tive action upon the sperm cells and may eliminate from fertili- 
zation the majority of the male-producing spermatozoa. The 
large female-producing sperm cells may show a greater rate of 
motility, greater endurance, or for some other cause out-distance 
the male-producing spermatozoa, thus resulting In a prepon- 
derance of female offspring in advanced service groups. 
A possible explanation for the high percentage of deaths 
among females lies in evidence showing that the percentage of 
female offspring is increased by heavy service of the male as 
shown on page 607. The weight (Minot, Jackson, King) of 
