22 



THE AMERICAN NATURALIST [Vol. LV 



the above mentioned stock, while the original males were 

 purchased from a local dealer. 



In 1911 a number of animals were selected from the 

 stock of the Experiment Station to start a control ex- 

 periment. The mating of even second cousins has been 

 avoided in this stock, which has been called Experiment B. 



Eighteen of the inbred families were still on hand in 

 1916, having then on the average about eleven genera- 

 tions of brother-sister matings back of them. At this 

 time most of the families were disposed of in order to 

 make room for cross-breeding experiments and to obtain 

 larger numbers from the five families which it was de- 

 cided to retain. These families, numbers 2, 13, 32, 35 

 and 39, are the ones which have been tested for resist- 

 ance to tuberculosis. They were retained in part because 

 they already occupied many pens, but largely because of 

 the possession of contrasting characteristics in size, fer- 

 tility and coat pattern. Each of these families is at 

 present very homogeneous in heredity. Family 2 is de- 

 scended wholly from one mating in the sixth generation 

 of inbreeding. Families 13, 32, 35 and 39 come from 

 single matings in the seventh, eleventh, twelfth and 

 eighth generations respectively. 



The total number of animals involved in the experi- 

 ment on inbreeding and cross-breeding has been about 

 30,000. 



The Effects of Inbreeding on Vigor 

 It is noteworthy that there has been no very obvious 

 decline in vigor, although the families are now on the 

 average in the fourteenth generation of brother-sister 

 mating and one of the most vigorous (No. 35) has 

 reached the tweiit y-lirst generation. 



There has, however, V)een some decline in vigor in all 

 respects which have been studied. The decline is most 

 marked in fertility, including both frequency and size of 

 Utters. It has been so great in this respect that it 

 would have to be recognized even though the decline in 



