EFFECTS OF INBREEDING AND CEOSSBEEEDING. 



21 



BIRTH WEIGHT. 



Figure 1 1 shows how the various experiments compare with respect 

 to birth weight. On the whole there is considerable similarity to the 

 situation with respect to percentage born alive (Fig. 8). In both 

 cases the records of inbred females, however mated (inbreds, CO, CA) 

 are poor compared with those of crossbred females (AC, CC, Cl). 

 Again there is a marked decline in both cases in the second generation 

 of renewed inbreeding. 



This similarity may be due in part to a direct causal relation. Still- 

 ,born young are naturally considerably lighter than young born alive. 

 The high birth weights of the progeny of crossbred dams might thus 

 be due merely to their relatively small percentage of stillborn young. 

 This complication is avoided by comparing the birth weights of only 

 those young which reach 33 days (Fig. 12). We find, in fact, that 



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Fig. 11.— The birth weight of all young born, 1916-1919. Indices correcting for effects of size of litter 

 and seasonal conditions (Table 2). (See Fig. 8 for explanation of symbols.) 



the differences among the experiments are reduced. Nevertheless 

 the essential points noted above are still present. It seems clear 

 that the prenatal rate of growth depends largely on the character- 

 istics of the dam as far as it depends on heredity at all. The heredity 

 of the young, however, seems to be more important than in the case 

 of the percentage born alive, as indicated by the records of Experi- 

 ments CO and CA in comparison with the inbreds. That the agree- 

 ment in the standing of the crossbreeding experiments in relation to 

 the total inbreds in the two respects, birth weight and percentage 

 born alive, does not rest on a common physiological factor is shown 

 by the lack of agreement among the separate inbred families. 



Experiment CG, in which the parents were selected because of 

 their exceptionally great weight at weaning, produced heavier young 

 than Experiment CL, in which the parents were underweight at the 

 same age. Even CL, however, is well above the average of the inbreds 



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