THE PRACTICE OF BREEDING 293 



improvement among the best strains of animals is so 

 difficult. Highly improved breeds or families among 

 the domestic animals have reached a degree of purity 

 wherein the germ-plasm represents an approach to homo- 

 geneity. When such a pure line has been established, 

 the old adage that " like begets like " becomes a really 

 working principle and a guide to practice. The novice 

 need not expect to accomplish great improvement in the 

 highly developed breeds of live-stock. The greatest 

 improvement will be made in breeds or strains of mixed 

 breeding, genetically speaking. 



The improvement of animals of mixed character which 

 will result from the efforts of a breeder is probably to be 

 explained as a gradual separation of the desirable pure 

 lines and mating the individuals which possess these 

 characters. This results in time in the establishment of 

 a strain or breed which is prepotent in transmitting the 

 desired qualities when after many generations of pure 

 line breeding this strain has reached a point where the 

 germ-plasm of the breeding animals is pure in respect 

 to its genetic factors ; then any considerable improvement 

 will be no longer possible. 



278. Pure line theory not opposed to improvement by 

 selection. The practical breeder whose experience has 

 demonstrated clearly that improvement among the regis- 

 tered or " pure-bred " races of the domestic animals has 

 followed careful and persistent selection will hesitate 

 to accept the statement of Johannsen and his followers 

 that " selection within the pure line is without effect." 

 But the improved breeds of live-stock, however pure 

 in breeding, are not to be regarded as fulfilling the require- 

 ments of a " pure line " in the biological sense. The 

 breeds of live-stock are yet far from homogeneous in 



