EFFECTS OF INBREEDING AND CEOSSBREEDING. 19 



than in CC and AC. Thus the dams with Uving young in the former 

 experiments were a more selected sample than those of the latter. 

 It seems probable, then, that there really is a relation between the 

 breeding of the dam and success in rearing the young, but that it is 

 masked in the present case by an interrelation with the mortality 

 at birth. 



The records of the inbred families are interesting in this connection. 

 There is considerable variation, which, as shown in the previous paper, 

 is significant and persists from year to year. The two poorest families 

 in rearing the young (39 and 32) were the two best in percentage alive 

 at birth, while the poorest in the latter respect (2) is next to the best 

 in rearing the young. There is here a suggestion of a negative inter- 

 relation (r=— 0.60). It has been shown, however (part II, Bul- 

 letin 1090), that in two earlier periods when 23 families were on hand 

 there was no significant correlation between the records in these two 

 respects (r=+0.03 for 1906-1910 and r= +0.30, 1911-1915). 

 As indicated in the above paper, we probably have an unstable balance 

 between the influence of certain genetic factors which tend to bring 

 about a positive correlation and the tendency toward a negative 

 correlation due to the influence of selective mortality at birth on post- 

 natal mortality. 



The record of Experiment Cl is somewhat below that of CC. It is 

 of course expected that a decline will be shown in the first generation 

 of renewed inbreeding in a character in which the breeding of the 

 young is a factor. If the breeding of the dam were of no importance 

 at all, the record of Cl should be only half as much above the inbreds 

 as CC. The relatively small decline of Cl, especially in view of its 

 unexpectedly high record in percentage born alive, is thus evidence 

 that the breeding of the dam does count. There is a further decline 

 in Experiment C2, where both parents and young are inbred. As 

 will be explained later, the record of C2, whether due to dam or young 

 or both, should be just halfway between the inbreds and CC. 



The records of CG and CL are somewhat lower than expected but 

 still above the best of the inbred families. Here again it is shown that 

 the condition of young at weaning gives no indication of theu' success 

 later as parents. 



The mortality between birth and weaning is shown separately for 

 each size of litter in Figure 21. The results agree with those obtained 

 from the use of indices. 



Summing up, the mortality between birth and weaning depends 

 primarily on the characteristics of the dam. A much larger percent- 

 age are raised among crossbreds in all sizes of litter than in the best 

 of the inbred families. The apparent thriftiness of the parents when 

 they themselves were weaned gives no indication of the mortality 

 to be expected among their young. 



