574 FRANK A. HAYS 
and was discarded; both No. 22 and No. 29 died after having 
given birth to but one litter upon which we secured data; and 
No. 18 died after she had given three litters to the experiment. 
Age of the dam is an important factor as affecting the number 
in the litter and probably to some extent the weight of the in- 
dividuals of the litter. For this reason the approximate ages of 
the breeding animals is here given in order that the reader may 
understand fully how much error may have been introduced 
through immaturity in the breeding females. One female gave 
birth to young when six months old; two, when seven; four, 
when nine; one, at ten, and one, at eleven months old. The 
remainder of the females was fully mature, that is, fifteen months 
old or over, at the time they gave birth to the first litter used in 
this experiment. 
The fact should be noted that the three females that died 
during the experiment were all immature at the time they first 
reproduced and that only one of them (No. 18) had more than 
one litter upon which we secured data. Two of the three lit- 
ters from No. 18 are 5th-service litters by male No. 3 and the 
other is a Ist-service litter by the same male. Female No. 22 
gave a single litter from the 5th-service by male No. 1; and 
female No. 29, one litter from the Ist-service by male No. 4; 
female No. 12 has contributed but two litters to the records; 
namely a Ist- and a 10th-service litter by male No. 1. 
It may appear to the reader that considerable error, resulting 
from the use of these immature females, was overlooked in mak- 
ing up our records of growth, but this has not been the case; 
therefore, a brief consideration of the system of matings used to 
overcome this error is not out of place here. 
The system of matings was arranged so that each female was 
mated to at least two of the males and many to all three males, 
and where possible each female produced litters from all differ- 
ent services from each male, thus reducing parental variability 
to the males alone. By making the three breeding groups of 
females as nearly equal as possible in age and weight; by distrib- 
uting the heavy service among the females in such a way as to 
secure all types of litters from both mature and immature fe- 
