18 



BULLETIN 1121, U. S. DEPARTMENT OF AGEICULTUEE, 



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 



86 



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Fig. 9.— The percentage raised of the young born ahve, 1916-1919. Indices correcting for effects of size 

 of htter and seasonal condition. (See Fig. 8 for explanation of symbols.) 



early age come to mind here. Experiment CO 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 



