28 BULLETIN 1121, U. S. DEPARTMENT OF AGRICULTURE. 
in CG produced a litter two weeks earlier than the small-sized 
animals of CL selected because of birth in a large litter. 
An unexpected result is shown in the last column in the long 
interval between mating and first litter when the animals were 
already 3 months or more of age at mating. Compare especially 
the age of 5.05 months at which weanlings in Experiment B pro- 
duced their first litter with the interval of 4.23 months between 
-mating and first litter in the case of parents already mature at the 
time of mating. 
COMPLETE STERILITY. 
There has not been much complete sterility in any stock, inbred or 
crossbred. Among 335 matings of inbreds which were kept for at 
least one year after maturity, only six failed to produce young. 
Three of these were kept through a second full year and continued 
sterile. Two of them belonged to Family 36, one to Family 17, 
while the other three were in Experiment CO, two being matings 
Bienes 
rama 
Lig Pe A 
earl oes 
et 
Fic. 17.—Average size of litter, 1916-1919. Effects of seasonal conditions eliminated (Table 2). (See Fig. 
8 for explanation of symbols.) 
LMT 
8 
SLPO C: 
between a male of 38 and a female of 13 and the other between a 
male of 39 and a female of 32. Among 234 matings in which one or 
both of the parents were crossbred and which were kept for more 
than 12 months, only two failed to produce young. 
SIZE OF LITTER. 
In the study of the size of litters the heredity of the young is again 
shown to be of no account by the comparison (Fig. 17) between the 
inbreds and the first cross (CO). The superiority of Experiment CA 
over CO is so slight (1.5 PF) that it can not be taken as evidence 
that crossbreeding of the sire has any influence on size of litter. 
The reciprocal cross, inbred male by crossbred female, however, 
gives such a great increase that there can be no question about the 
effect of crossbreeding of the dam on her fecundity. 
The effect of crossbreeding of one of the parents, presumably the 
dam, is shown by the records of Experiments CC and C1. But the 
