42 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



the same is true of continued self-pollination in plants as brought 

 out by Darwin, Shull, East, Jones, and Collins and Kempton. 



Summing up, the primary effect of inbreeding is the automatic 

 fixation of some combination of hereditary factors present in a stock. 

 This leads to uniformity of type or function, if such uniformity is 

 possible. When not possible, owing to variability which is not 

 genetic, the hereditary potentialities in the lines are brought out 

 clearly, as is possible in no other way. Decline in vigor is a co mm on 

 but not a necessary consequence of the fixation of heredity. 



CROSSBREEDING. 



As inbred animals produce only one kind of reproductive cell, it is 

 to be expected that a cross between two inbred lines will produce 

 only one kind of progeny so far as hereditary factors are concerned. 

 It is, in fact, well known that the first cross between two closely 

 bred stocks is as uniform in character as either of the parent stocks. 

 The cross between the polled black Aberdeen-Angus breed and 

 white Shorthorns, producing polled blue roans, has already been 

 considered. The conformation is also uniform in the first generation. 



While uniform themselves, such crossbreds are anything but 

 prepotent as breeders. When two of the above-mentioned polled 

 blue-roan Shorthorn-Angus crosses are bred with each other, the 

 progeny, as already mentioned, include blacks, reds, blue roans, red 

 roans, and whites with either black or red ears. Any of the colors 

 may be associated either with horns or polled heads. There is also 

 increased variability in conformation. 



As regards vigor and fertility, crossbreeding is likely to lead to 

 marked improvement. As noted above, the crossing of two unrelated 

 weakened inbred lines usually leads to a return to normal vigor. 



Summing up, the first generation of a cross is as uniform in charac- 

 ter as the parent stocks, and in general shows increased vigor. In 

 the second generation there is increased variability, the characteristics 

 of the grandparents being combined, as a rule, hi all combinations 

 and in all degrees. 



THE SYSTEM OF BREEDING. 



THE PURPOSES OF LIVESTOCK BREEDING. 



Aside from mere increase in numbers, the purposes which the 

 breeder is likely to have in mind fall under two more or less distinct 

 heads, namely, production of a uniform product and improvement. 

 A uniform product depends on such control over the heredity of the 

 stock that matings can be made with the assurance that the offspring 

 will be of a certain definite type for which there is a demand. Im- 

 provement is, of course, closely related to control over heredity, but 



