248 THE BREEDING OF ANIMALS 



But when it is attempted to perpetuate the superior 

 qualities of the cross-bred animal by breeding, disappoint- 

 ment invariably results. The second cross resulting 

 from the mating of two cross-bred animals may be totally 

 unlike either of the immediate parents or of the original 

 pure-bred forms. Crossing, therefore, is not a method to 

 be employed for rapid improvement or for fixing desir- 

 able qualities. It is opposed to in-breeding which does 

 increase prepotency and is the most rapid method known 

 of fixing desirable characters. 



233. Cross-breeding as a cause of variation. The 

 fact that crossing disturbs the balance of characters and 

 brings about recombinations in the germ-plasm gives it a 

 peculiar value in causing variations to appear. The 

 breeder who is working with pure-bred animals which owe 

 their purity of breeding to a long period of careful selec- 

 tion by skillful breeders cannot hope to cause any great 

 degree of improvement. Pure-bred animals are already 

 improved. About all any breeder working with pure- 

 bred animals can do is to select out the highly desirable 

 strains from those of lesser value already in the breed. 

 But as Johanssen has shown, there are very definite limits 

 beyond which the improvement of pure lines cannot go. 

 Marked improvement must come through variation. 

 Crossing is a common cause of variation. Variations 

 which appear as the result of crossing may be desirable 

 or undesirable. They may be relatively unimportant 

 or they may be in the nature of a valuable mutation. 

 Such valuable mutations may be perpetuated by in -breed- 

 ing and a new and valuable quality sometimes secured 

 in this way. This method is not practical for breeders 

 of registered live-stock under present conditions, but has 

 been of great service to the breeders of plants. 



