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THE AMERICAN NATURALIST [Vol. XLIII 



Selection indices of the character above described may 

 be separated into two general categories. These cate- 

 gories may for the sake of convenience be designated as 

 (a) fundamental and (b) special. A selection index is 

 to be regarded as fundamental when it includes in its 

 formula those characters of an organism which every 

 breeder will wish to maintain at a high grade or improve, 

 whatever may be the special purpose for which he is 

 breeding. Thus, for example, in maize breeding whether 

 the breeder is working for high protein or high oil con- 

 tent or earliness, he will always desire to have well-shaped 

 ears with good tips and of good size, a high yield of 

 shelled corn to the acre and good vitality (i. e., germi- 

 native capacity) in his seed. 



These and other characteristics which we need not take 

 space here to enumerate are in a sense fundamental or 

 universal characteristics which every breeder wishes to 

 improve or, at least, to keep at a high standard. Similar 

 considerations obtain in breeding dairy cattle or poultry 

 for performance. Besides the performance in respect 

 to milk production or egg production every breeder of 

 these animals desires that what may be called the " breed- 

 ing capacity" of his stock shall be constantly improved 

 or, at least, not impaired. By 1 'breeding capacity" is 

 meant ability to produce numerous and vigorous healthy 

 offspring, to put the matter in the broadest way. An 

 index number which has to do with these universally de- 

 sired characters may be called the fundamental selection 

 index for each type of organism. 



On the other hand, by a special selection index is to be 

 understood one which includes those characteristics which 

 a breeder is specially and particularly working to im- 

 prove. In the case of the corn breeder working for high 

 protein, the protein content of the ear will constitute one 

 of his special selection indices either by itself or in com- 

 bination with some other characters*. In dairy cow breed- 

 ing the butter fat content of the milk will be a special 

 selection index. 



