2 BULLETIN 1121, U. S. DEPARTMENT OF AGRICULTUEE. 



come to be distinctly inferior to a control stock in all of these respects 

 and also in resistance to tuberculosis. It was concluded from a 

 consideration of the various lines of evidence that the inbreeding 

 was in some way responsible for at least a part of the decline. 



In Part 11^ a detailed study was made of the 23 separate inbred 

 families. It was shown that marked hereditary differentiation 

 had been brought out among them early in the course of the inbreed- 

 ing and that the differences had increased later. There had been 

 also an automatic differentiation and fixation of the more obvious 

 characteristics, such as color, pattern, and tendency toward poly- 

 dactylism and toward the production of particular types of monsters. 

 It was found that the various elements of vigor and weakness had 

 become fixed in almost every possible combination in the various 

 families, there being no evidence for hereditary differences in general 

 vigor. 



The purpose of the present paper is to present the results of crosses 

 between the inbred families. 



THE INBRED FAMILIES. 



Of the 17 inbred families still in existence in 1916, only 5 have been 

 retained to the present. It seemed necessary to eliminate the others 

 in order to make room for the crossbreeding experiments and to 

 obtain sufficient numbers from these five. Families 2, 13, 32, 35, 

 and 39 were the ones retained, partly because they occupied a large 

 number of pens, and partly because of contrasting characteristics. 

 Family 39, for example, had the least white in the coat, while Family 

 13 had the most. Family 32 had a peculiar intense golden agouti. 

 Families 2 and 13 were at opposite extremes in weight and also 

 contrasted in size of litter. 



CROSSBREEDING EXPERIMENTS. 



The control stock, Experiment B, has been maintained, as from the 

 first, by matings between individuals less closely related than second 

 cousins. 



Since 1916 a large number of first crosses have been made between 

 different families. These constitute Experiment CO. The young are 

 crossbred but the parents are inbred. 



Some of the progeny from Experiment CO have been mated with 

 others from a different cross, thus bringing together four inbred 

 families. This is Experiment CC. Both parents and young are cross- 

 bred in this case. 



Other animals from Experiment CO have been mated brother with 

 sister. This is Experiment Cl. The parents are crossbred but the 

 young are to some extent inbred. 



2 U. S. Department of Agriculture Bulletin 1090. 



