SEXUAL ACTIVITY OF MALE RABBITS 599 
with the study of the weight graphs than in average body weight 
the advanced service litters are equal and in some cases superior 
to that of the litters in the light service groups. The fact that 
variability within litters is small compared with the variability in 
service groups is well illustrated by a comparison of the coeffi- 
cients in tables 3 and 4. 
4. Growth by measurements as related to frequency of copulation 
Charts 14 to 21 are presented to show the growth in the mean 
dimension as obtained on forty-five litters. The method of mak- 
ing measurement and the determination of the mean dimension 
have been already explained, pp. 581-582. Each graph repre- 
sents averages of the mean dimension for all litters of the same 
size in the respective service groups. The mean dimension for a 
litter is obtained by adding all head measurements to all meas- 
urements of ilial extremes and dividing the sum by the total 
number of readings included in the sum. The expression thus 
obtained is the average individual mean dimension for the re- 
spective litters and may be compared with the average individual 
weights used in the previous charts. 
These charts of body development show that there is a maxi- 
mum increase in the mean dimension up to about the twentieth 
day, after which there is a very noticeable flattening of the 
graphs. From about the twentieth day on to the end of the 
observations at ninety days the progressive increase in the mean 
dimension is about constant. The increase in the mean dimen- 
sion is thus in marked contrast to the increase in body weight 
previously illustrated by charts 1 to 138. Body weight has been 
shown to make a rather constant increase up to the end of ninety 
days, and this is well illustrated by the fact that the weight 
graphs show little if any tendency to flatten out. 
Though the number of litters making up a mean dimension 
graph is in most cases small, they serve to illustrate the same 
principle as the weight graphs, namely, that the advanced serv- 
ice progeny are fully equal to the Ist- or 5th-service progeny 
at all times during the ninety days of the observation. On 
