18 BULLETIN 1121, U. S. DEPARTMENT OF AGRICULTURE. 
generation of renewed inbreeding. The apparent thriftiness of the 
females in their early life gives no clue to their later success in bearing 
young. 
THE PERCENTAGE RAISED AMONG THE YOUNG BORN ALIVE. 
On comparing Figure 9 with Figure 8, which we have just been con- 
sidering, the difference in the positions of Experiments CO and CA at 
once attracts attention. In fact, as regards the percentage raised of 
the young born alive, every crossbreeding experiment is above the 
average of the best inbred families. It is clear that the breeding of 
the young is an important factor in the mortality between birth and 
weaning. The relatively advanced stage of development at which 
guinea pigs are born and their ability to care for themselves at a very 
80 
PLEA? CENT 
a 
70 
oS 
Fig. 9.—The percentage raised of the young born alive, 1916-1919. Indices correcting for effects of size 
of litter and seasonal condition. (See Fig. 8 for explanation of symbols.) 
early age come to mind here. Experiment C0 is like the inbreeding 
experiments as regards breeding of sire and dam and also as regards 
the elimination of young at birth. The improvement of CO over the 
inbreds shown in Figure 9 thus measures directly the effect of the 
crossbreeding of the young themselves. The level of random-bred 
stock is reached immediately. The record of Experiment CA, also | 
inbred dam, crossbred young, is even higher. The fact that the | 
records are no higher in Experiments CC and AC, in which the young | 
are equally crossbred and the dam also is crossbred, might be taken as | 
indicating that the breeding of the dam is of no importance whatever | 
in the rearing of the young. This conclusion, however, is not really | 
warranted by the data. It will be remembered that there was a | 
distinctly heavier elimination at birth in Experiments CO and CA | 
