EFFECTS OF INBREEDING AND CROSSBREEDING. 51 



Family 2 was the poorest family in producing litters between 1906 

 and 1910, but tied with Family 23 for the first place in the second 

 period. The change was nearly as great in Family 35. The cor- 

 relation becomes + 0.66 if these two families are omitted. 



Looking through the table, we find that Families 35 and 2 made 

 conspicuous changes in rank in several other respects. A few other 

 families also made numerous changes. In fact, a majority of the 

 marked cases of relative improvement or deterioration are found in 

 Families 35, 2, 24, 20, and 21, and these include 10 of the 13 cases 

 in which there were changes of more than two grades. There were, 

 on the other hand, six families (1, 9, 11, 13, 17, and 18) which made 

 no important changes and eight more with only one or two changes. 

 We are thus led to look into the history of the families to see why it 

 is that certain of them have changed in many respects while others 

 have remained constant. 



Careful study of the pedigrees shows that there has been a much 

 greater revolution in the predominant lines of descent in Family 35 

 than in any other family. A single mating was made in the first 

 generation of this family, but four were made in the second genera- 

 tion, which may be looked upon as founding four subfamilies. The 

 smallest of these subfamilies, which during the first seven generations 

 included only 17 per cent of the matings, suddenly began to expand 

 at this time and produced 65 per cent of the matings made between 

 the eighth and twelfth generations. All of these were descended 

 from a single mating in the seventh generation. By the end of 1917 

 the entire family, which was one of the largest in the stock, was 

 descended from a single mating in the twelfth generation and had 

 reached the most advanced generation of inbreeding in any family. 



With this history, it can hardly be a coincidence that Family 35 

 has changed in character more than any other family. It will be 

 seen from Table 5 that Family 35 made poor records during the first 

 period in birth weight, gain, and weight at 33 days, in size and fre- 

 quency of litter, and hence in total fertility, and finally in the per- 

 centage raised of those born alive. During the second period the 

 birth weight, gain, and weight at weaning were good; size and fre- 

 quency of litters and total fertility were among the best, and the 

 percentage born alive and the total percentage raised were good. A 

 decline from rank B to C in the percentage born alive meant merely 

 a change from eighth to ninth among the 22 families. Following 

 the second period, i. e., since 1915, the relative improvement con- 

 tinued in every respect and the family became on the whole 

 the most vigorous in the stock, displacing Family 13, wliich had 

 previously held this position. Family 35 evidently started with 

 relatively inferior heredity for most elements of vigor. Apparently, 



