248 APPLICATION OF BREEDING PRINCIPLES 



The chief advantage of inbreeding is the possibility which it 

 offers of fixing or making permanent the blood of some valuable 

 individual. In-bred progeny are exceptionally potent. Another 

 advantage is that it avoids the introduction of new blood which 

 might produce objectionable characteristics. Inbreeding is the 

 greatest force known to intensify existing blood lines, and this 

 factor makes it one of the best as well as the worst system to use; 

 for it intensifies all characteristics of the individual, whether bad 

 or good. Therefore the breeder who resorts to this method of 

 mating must exercise special care in eliminating objectionable 

 factors which would be intensified equally with the good ones. 

 It is often supposed that inbreeding tends to reduce vigor and 

 vitality; but, when this is the case, it is undoubtedly due to the 

 fact that these characteristics exist already and are intensified in 

 the progeny. If poultry breeders were more careful in selecting 

 for vigor and vitality, this apparent objection to inbreeding would 

 be overcome and less heard of. 



Line Breeding. Line breeding may be defined as the breeding 

 of individuals which are selected from, or restricted to, a single 

 line of descent. For example, it is the process of breeding within 

 one family or within a limited number of families all of which 

 have a common ancestry and represent similar types (Fig. 126). 



Line breeding offers good opportunity for improvement, since 

 it excludes everything outside of the chosen line of descent and 

 combines in the progeny the characteristics especially desired. 

 The result is the rapid purification of the pedigree- and the fixing 

 of a type. There is slight danger of outside or alien traits appear- 

 ing. This is a very conservative system of breeding, and is the 

 one practised by many of the best poultry breeders. It has resulted 

 in building up some of our best strains of standard-bred poultry. 

 The leading advantages of line breeding are two: (1) The probable 

 certainty with which results may be predicted, thus allowing the 

 breeder to work with his eyes open; (2) the progeny of line-bred 

 birds are backed up by a strong hereditary influence which results 

 in hastening improvements in the one desired direction, owing to 

 the lack of alien or mixed blood. 



In practising line breeding there is one point of caution, 

 namely, the necessity of making the matings both from the pedi- 

 gree records and by individual selection in the pens. Some breeders 

 are apt to neglect the latter factor of individual condition, and 

 make their matings from paper only; in consequence, a few genera- 



