30 BULLETIN 1121, U. S. DEPARTMENT OF AGRICULTURE. 
sires) from large litters (CL) or and small litters (CG) is without 
effect. 
TOTAL FERTILITY. 
The number of young born per mating year (Fig. 18) is merely the 
product of the two elements of fertility, frequency and size of litter, 
Fic. 18.—Young per mating per year, 1916-1919. Effects of seasonal conditions eliminated (Table 2). 
See Fig. 8 for explanation of symbols.) 
which we have discussed separately. It is interesting to find that 
the dam has more influence on total fertility than the sire, in spite 
of the greater influence of the latter on frequency of litter. Selection 
Fic. 19.— Young raised per mating per year, 1916-1919. Effects of seasonal conditions eliminated (Table 
2). (See Fig. 8 for explanation of symbols.) ; 
of parents on the basis of size of litter is shown to be wholly without 
effect. 
The best single measure which we have of the reproductive efficiency 
of the experiments is the number of young raised per year. This is 
the product of the young born per year and the percentage raised to 
33 days (actual, not index). Figure 19 shows the tremendous advance 
