236 PRINCIPLES OF POULTRY BREEDING 



about by an alteration in one or more of them. This variation 

 is not caused by, nor is it the result of, the introduction of an 

 absolutely new character. 



The fact that individuals do vary makes improvement possible 

 through selection and breeding which would otherwise be impos- 

 sible. Indeed, without this factor there would be no chance for 

 either improvement or deterioration; the type would be fixed in 

 all its characteristics. 



Variations are of two kinds, those which increase the useful- 

 ness of the individual and those which are undesirable or create 



FIG. 120. Barred Plymouth Rock chicks from the same mating and of the same age, 

 showing morphological variation. 



an inferior condition. It is the duty and purpose of the breeder 

 to select and intensify desirable variations whenever possible. 



Types of Variation. The unit of variability is not the individ- 

 ual, but the breed; and different individuals in the same group 

 are made up of different combinations of the same characteristics. 

 The real unit of variation is, then, the racial characteristic, but 

 the individual must be made the unit of selection. Four distinct 

 types of variation have been found, and a clear perception of 

 these is necessary for the intelligent study of the causes and behav- 

 ior of variations. These types are (I) morphological, (2) sub- 

 stantive, (3) meristic, and (4) functional. 



Morphological variation has to do with differences in form or 



