THE AMEBIC AN NATURALIST [Vol. LV 



The high resistance of one of these families was trans- 

 mitted by each sex to the offspring of each sex in crosses 

 with other inbred families. 



In crosses involving this most resistant family, the 

 progeny were superior even to this family itself, indicat- 

 ing not only the dominance of resistance over suscepti- 

 bility but possibly also the presence of complementary 

 factors among the families. 



In crosses among the more susceptible families, the 

 progeny were little, if any, superior to the family of the 

 better parent, indicating dominance but not complemen- 

 tary factors in this case. 



Over 30 per cent, of the variation in length of life after 

 inoculation among the crossbreds was determined by the 

 amount of blood from the best inbred family. Allowing 

 10 per cent, of the variation as due to age, weight and 

 condition, 50 to 60 per cent, remains as due to accidental 

 and unknown causes. 



The factors which determine the resistance of a family 

 to tuberculosis are not closely related to the other ele- 

 ments of vigor, including rate of growth and adult 

 weight, frequency and size of litter, the percentage of the 

 young born alive and the percentage of these raised to 

 weaning. There is some evidence that they are contribut- 

 ing factors to a sufficient extent to give the most resistant 

 family a record above the average in each element of 

 vigor and so give it the highest or nearly the highest 

 total efficiency. Even this relation of resistance to other 

 elements of vigor appears to be present only under ex- 

 ceptionally adverse conditions when it may play a direct 

 part in determining the health of the stock. 



The results in regard to resistance to tuberculosis are 

 like those for other characteristics as regards the differ- 

 entiation among families brought out by inbreeding, the 

 improvement resulting from crosses between inbred 

 families and the independence genetically from other ele- 

 ments of vigor. 



