EFFECTS OF INBREEDING AND CEOSSBEEEDING. 33 



On the other hand, marked differences were found among the inbred 

 families. Family 35, the most resistant, lived about twice as long 

 after inoculation, on the average, as the poorest family, No. 39. 

 Family 2 is second in resistance. Family 32 third, and Family 13 

 fourth. 



Families 35 and 2 are superior to the random-bred stock B in this 

 respect. 



Crosses between families produce young which are in general at 

 least as resistant as the better parent family. In particular cases 

 the young are distinctly more resistant than either parent family. 

 There is thus dominance of resistance over susceptibility and in 

 particular cases each parental family supplies something lacking in 

 the other. 



Comparisons between Experiments CO, CA, and AC indicate that 

 the full effect of crossbreeding is manifest in the first generation. 

 The degree of resistance is determined by the genetic composition 

 of the animal itself, the sire or dam being without direct influence. 



EARLY VIEWS ON INBREEDING. 



The general character of the results which have been obtained in 

 the present experiments have little novelty. Inbreeding has been 

 practiced by innumerable livestock breeders, in some cases merely 

 because it was the path of least resistance, in other cases deliberately. 



The great majority of these breeders, even those of the second 

 class, have undoubtedly seen something like the degeneration de- 

 scribed in the present paper — reduced size, lowered fertility, and 

 increased difficulty in raising the young. In many cases much more 

 serious degeneration has been encountered and the inbred line has 

 rapidly become extinct. Conversely, the beneficial effects of out- 

 crossing have often been observed. The popularity of the Angus- 

 Shorthorn cross in Scotland threatened at one time to wipe out the 

 pure Angus breed. 



On the other hand, the use of inbreeding was an essential element 

 in the success of the noted breeders, who laid the foundations of the 

 modern pure breeds of livestock. It was to a large extent by in- 

 breeding from carefully selected animals that they fixed the type 

 which they desired and made it prepotent. This effect as well as 

 degeneration finds its parallel in our experiments. We may call 

 attention here to the unconscious fixation of color and to the isola- 

 tion of important genetic dift'erences in characters, such as weight and 

 fertility, in which the. degree of determination by heredity is too small 

 to furnish a handle for direct selection. 



Breeders thus have obtained sufficiently definite consequences 

 following inbreeding. The question as to the effects has remained 



