EFFECTS OF INBREEDING AND CROSSBREEDING. 27 
selection from a group of inbred lines, followed by crossbreeding, 
is clearly indicated. The method is especially applicable to characters 
such as those used here, in which heredity is such an insignificant 
factor in individual cases that selection of individuals is of no value. 
AGE OF MATURITY. 
It was thought at first that the influence of the sire on frequency 
of litter as here measured might be due to earlier maturity of crossbred 
males. Females, as noted previously, may become capable of repro- 
duction at one month of age, while males only begin to mature at 
about two months. If crossbred males mated at weaning were able 
to produce a litter in advance of the inbred males, they would produce 
more litters in the first year of mating even though there were no 
greater regularity after their first litter. However, a tabulation of 
the ages of the males at birth of their first litters (Table 10) shows that 
earlier maturity of the males does not explain the superiority of CA 
over AC. Their first litters were produced at nearly the same age, 
(5.09 and 5.03 months). The superiority of all crossbreeding experi- 
ments (except CO) over the inbreds seems, however, to be in part ex- 
plained by earlier maturity of the parents. 
TABLE 10.—Average age of males, mated when 1 or 2 months of age, at birth of their first 
litter, and average interval from mating to first litter in case of males 3 months old or 
over when mated. First litters born 1916-1919. 
fale mated at Male mated at Male 3 months 
b 
| 
| 1 month. 2months. | or over. 
Experiment. Average | Average | Average 
\Num-| ageat Num-| ageat |Num-) interval 
‘ber. | first ber. | first’ { ber. | to first 
litter. litter. | litter 
——ooe | = 
=k See I Sas ee eet Pee eee | 62 5. 84 1 5.00} 5 3.20 
2 a Le Re SEES CORRS Se IDR! | 55 ACT Ra eee oe | 2 3.00 
Sipe ee ee eee a ee eee ee ee Oe eee ; 40 5. 78 1 5.00 | 3 4,00 
i le A ae Ase SRE ee | 76 EES fae i ee obey | ee 
Lipseens =o ee a eee eee ee 25 6. 28 is Mea ere 6 4, 50 
LL LET Ore 2 bain ib iat AUS ia a Sg ha ap ole Lad 84 | 6.18 aaa areca Onite2 eeceeee 
epee) eile oy) tire FG ole: 342 KG5he 2 5.00) 18 4.00 
Laon Seer ae 2 Boe ee ee ee ee eee | 94 5. 73 33 6. 06 30 4.17 
eile le Wah) £5 ares a Th ee ds OS reise | 38 5. 09 | 4 5. 50 3 4. 33 
ah See SEE YS PS ee ed oe en ee i; 30 5. 03 7 5.43 9 3. 67 
Sri aa ig? Sen sitar fire Bon | 20 4.90 20 5.40} 4 3. 50 
pa RE EO, EAE Eat ces oes ae en eam 46 5. 09 5 5. 80 | Gils: oes 
Pe eo oe ane ete teh eer 2 eu 17 4.77 2 5. 00 | ed He eee 
oe 0) ES SES 2 ee Bae eee | 29 5. 21 6 5.17} 4 3. 25 
Loe So iia a ey ae | 40 4.68 10 5. 20 (ie ee ee apc 
sek Deb Sale ems arene aerate | 20 5.05 | 29 5.48 | 48 4,23 
Apparently the females, which were in all cases of practically the 
same age as the males at the first litter, were also in part responsible 
for the delay in the first litter of inbreds. The relatively early age 
of first litter in AC proves this. 
One interesting side light is the remarkably early average age at 
first litter in Experiment CG (4.68 months) and the greater age in 
CL (5.21 months) as compared with most of the other crossbreeding 
experiments. We have here the clearest case of a difference between 
these experiments due to selection. The unusually heavy animals 
