20 Farmers' Bulletin 1167. 



ability characteristic of the second generation of a cross. Only by 

 breeding within relatively narrow limits can one be reasonably sure 

 that he is mating animals which both look alike and have the same 

 heredity. 



INBREEDING. 



Next in importance to selection as a means of improvement in 

 breeding animals, the most effective process at the command of the 

 breeder is the judicious mating of related animals. This process is 

 known as inbreeding, and various terms, such as line breeding, close 

 breeding, and incestuous breeding, have been used to define varying 

 degrees of intensity of the process. 



Jersey bull, Pogis 99th of Hood Farm No. 94502. 



FIG. 12. Animals such as this one show strong evidence of breed character, mascu- 

 linity, and impressiveness. 



Inbreeding is one of the most discussed subjects in the whole 

 field of genetics. All sorts of bad results are attributed to it. Lack 

 of vigor, nonresistance to disease, decline in size and fecundity, and 

 even sterility are the fate of inbred animals, in the minds of many 

 people. These ideas have a certain basis in fact, and often extend to 

 farmers, who sometimes refuse to buy males which have inbred pedi- 

 grees. Breeds and families which have been intensely inbred for sev- 

 eral generations develop manifestations of constitutional weakness 

 which, it seems, can be explained only on the grounds of their close 

 relationship. Scientific data of exact and elaborate scope, worked 

 out under controlled conditions, are not abundant on this point. 



