58 BULLETIN 1090, IT. S. DEPARTMENT OF AGRICULTURE. 



the origin of the monsters. Inbreeding seems merely to have brought 

 to light genetic traits in the original stock. 



Although most of the families came from the same line-bred stock, 

 a striking -differentiation with respect to traits connected with vigor 

 was found among them. These traits included size and frequency of 

 litters, percentage born alive and raised of those born alive, birth 

 weight, and gain to 33 days. The differences between the families 

 were greater than could be due to chance, and increased as the in- 

 breeding progressed and the families became more homogeneous 

 through the elimination of early branches from the main lines of 

 descent. The families tended to keep the same rank with respect to 

 each character. The correlation between the average grades in the 

 early and later histories of the families was high in respect to size 

 of litter, birth weight, and gain. It was high enough to be significant 

 in the case of the percentage born alive. The correlations were 

 positive but of doubtful significance for the percentage rrised of the 

 young born alive and for the frequency of litters. A detailed study 

 of the individual families, however, showed that the correlations would 

 have been higher and all would have been significant but for two or 

 three families in which there had been reversal of relative importance 

 among the subfamilies and in which therefore a change in rank in all 

 or many respects was not surprising. Recent evidence indicates 

 that even these families have now become fixed in their characteris- 

 tics. The conclusion seems warranted that there was heredity of 

 all of the traits studied. 



There did not, however, appear to be heredity of general vigor. 

 The average vigor of a family in one respect was found to be in the 

 main independent of its vigor in other respects. Thus the average 

 success of the families in raising their young was not correlated with 

 weight or with size or frequency of litters. Neither was weight cor- 

 related with regularity in producing litters. There was not even a 

 significant correlation between the percentage born alive and the 

 percentage of those raised, although success or failure in each separ- 

 ately was undoubtedly characteristic of families. Similarly there was 

 no correlation between the average size of litter and litter frequency. 

 The only apparent exception, outside of high correlations between 

 birth weight, gain, and year weight, was in a high correlation between 

 weight and size of litter, for which there is undoubtedly some indi- 

 cation of a physiological interrelation. 



The study of the individual families brought out interesting ex- 

 amples of extreme vigor throughout the history of a family in certain 

 respects, associated with extreme weakness in others, as well as cases 

 in which all kinds of vigor or all kinds of weakness were combined. 



