EFFECTS OF INBREEDING AND CROSSBREEDING. 19 
than in CC and AC. Thus the dams with living 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 C1 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 afactor. If the breeding of the dam were of no importance 
at all, the record of C1 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 their 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. 
