EFFECTS OF IISTBREEDING AND CROSSBREEDIJ^G. 



/9/<S /^/T /S/e /9/9 



Fig. 3. — The average gain between birth and weaning (33 days) in 

 the inbred stock during successive 3-nionth periods, 1916-1919. 



duration) we obtain the desired estimate for inbreds producing litters 

 simultaneously with Experiment Cl. 



In dealing with other characteristics, the number of matings present 

 in each 3-month period does not necessarily provide the proper 

 weights to be applied 

 to the records of the 

 inbreds. Litters per 

 year, young per year, 

 and young raised per 

 year are properly 

 weighted by the num- 

 ber of matings, but in 

 the case of size of lit- 

 ter the number of lit- 

 ters produced by the 

 given experiment in 

 each 3-month period 

 (Table 23) should be 

 used. The percentage 

 of young born alive, 

 the percentage raised of all bom, and the birth weight of all born should 

 be weighted by the number of young born in each 3-month period as 

 given in Table 24. There is a slight impropriety here, owing to 

 the use of indices for these characters, but it is of no practical impor- 

 tance. Similarly the percentage raised of the young born alive is to 

 be weighted by the numbers born ahve, shown in Table 25. The 



birth weight of the 

 young raised, the rate 

 of gain to weaning, and 

 the weight at weaning 

 are to be weighted by 

 the numbers weaned 

 in each period, shown 

 in Table 26. The 

 estimated record of 

 the inbreds, simul- 

 taneous with each ex- 

 periment, is shown 

 for each character in 

 Table 28. 



This method of cor- 

 recting for the seasonal fluctuations rests on the assumption that 

 the condition of the total inbreds was genetically constant during 

 the four years. Theoretically one would expect practical constancy 

 in each family after a dozen generations of brother-sister mat 



w 



~ A? '^-e T^S /0-/e,J-3 4-6 ^ 7-9 /a-/ZjJ-J 4^-6 7-9 /0-/2, s/-3 4-6 ?-& MHZt 

 /9/6 /9/7 7§73 7S7h 



Fig. 4. — The average number of litters produced per year by mature 

 matings in the inbred stock during successive 3-month periods, 

 1916-1919. 



