16 



BULLETIN 1121, U. S. DEPARTMENT OF AGBICULTUBE. 



first cross, though marked improvement in the next generation. The 

 percentage bom alive, birth weight, size of litter, and frequency 

 of litters come here. In the cases of percentage raised of those born 

 alive, and gain, there is on the other hand marked improvement in 

 the first cross. 



THE PERCENTAGE BORN ALIVE. 



A number of points of interest are brought out in Figure 8 in refer- 

 ence to the mortality at birth. It is shown that there is considerable 

 variation among the inbred families, but that no one of them has as 

 good a record as the random-bred stock (B). There is only a slight 

 improvement, too small to be relied upon, when two families are 



90 



86 



\ 



7S 



B. 



f 









ftC / 



CI 



; 



:CZ 



39 



32 



01 

 35 



/3 



j^ 





* 



■■'t 





^ 



^s. 



84.6^6- 

 ,CG 



ca 



^^ 



^ 

 * 



X 



^^^^,0^ 





W 









T?.?*^ 



^ 



> 













a 



















Fig. 8. — The percentage bom alive, 1916-1919. Indices correcting for effects of size of litter and seasonal 

 conditions: 2, 13, 32, 35, 39, inbred families; 01, other inbred families; A, average of all tnbreds; CO, first 

 cross between inbred families; CA, crossbred male from CO, unrelated inbred female; AC, inbred male, 

 unrelated crossbred female from CO; CC, crossbred male, unrelated crossbred female; Cl, crossbred 

 brother and sister from CO; C2, brother and sister from Cl; CL, selection for large Utters, small weight; 

 CG, selection for small litters, large weight; B, random-bred stock. 



crossed (CO). When, however, the crossbred females produce young, 

 whether mated with an unrelated crossbred (CC), with a brother 

 (Cl), or with an unrelated inbred (AC), there is a substantial im- 

 provement which goes beyond the best of the inbred famihes and 

 reaches practically the level of the random-bred stock. In contrast 

 with the good record of Experiment AC is the relatively small im- 

 provement over the inbreds in the reciprocal cross CA, in which 

 inbred females are mated with unrelated crossbred males. It is clear 

 from these results that crossbreeding mcreases the number of young 

 born ahve, but that the breeding of the dam, not that of the young 

 themselves, is the critical factor. The most significant line of cleavage 

 between high and low records falls between Experiments A, CO, and 



