EFFECTS OF IjSTBEEEDIIS^G 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 ivhen 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 horn 1916-1919. 





Male mated at 

 1 month. 



Male mated at 

 2 months. 



Male 3 months 

 or over. 



Experiment. 



Num- 

 ber. 



Average 

 age at 

 first 

 litter. 



Num- 

 ber. 



Average 

 age at 

 first 

 litter. 



Num- 

 ber. 



5 

 2 

 3 

 2 

 6 

 



Average 



interval 



to first 



litter. 



2 



62 

 55 

 40 

 76 

 25 

 84 



5.84 

 5.80 

 5.78 

 5.40 

 6.28 

 6.18 



1 

 

 1 

 

 

 



5.00 



3.20 



13 



3.00 



32 



5.00 



4.00 



35 



5.50 



39 





4.50 



other. . 













Inbred 



342 

 94 

 35 

 30 

 20 

 46 

 17 

 29 

 40 



5.85 

 5.73 

 5.09 

 5.03 

 4.90 

 5.09 

 4.77 

 5.21 

 4.68 



2 

 33 

 4 

 7 

 20 

 5 

 2 

 6 

 10 



5.00 

 6.06 

 5.50 

 5.43 

 .5.40 

 5.80 

 5.00 

 5.17 

 5.20 



18 

 30 

 3 



4.00 



CO 



4.17 



CA 



4.33 



AC 



9 ! 3.67 



cc 



4 ! 3.50 



CI. . . . 



1 



C2 







CL 



4 1 3,25 



CG 



' 









B 



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 



