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THE AMERICAN NATURALIST [Vol. XLIII 



variety. Corn index numbers, as formulated above, are 

 only directly comparable within a single variety. They 

 may of course just as well be used in judging a dent corn, 

 for example, as for the sweet corn here discussed. The 

 average ear of dent corn will obviously give a different 

 value for the index from that given by the average ear of 

 sweet corn. 



Any one familiar with the score cards used in judging 

 corn in the various agricultural shows and fairs in this 

 country will recognize that the corn selection index form- 

 ula here given combines in one expression quantitative 

 determinations of a majority of the characters which ap- 

 pear on the score card. The index in a way takes the 

 place of the judge. It impartially ' ' cuts for each defect" 

 according to a previously agreed upon system of weights. 

 By the combined use of the index number, scales and 

 measuring tape, unconscious or conscious partiality and 

 bias is inevitably and absolutely taken out of the judging. 

 The selection index idea seems capable, when properly de- 

 veloped to meet the needs of particular cases, of supply- 

 ing in some measure that thing which has been so long 

 desired in all kinds of stock judging, whether of plants 

 or animals, viz., an absolute base or datum plane. 



V. Summary 



The purpose of this paper is to call the attention of 

 those interested in breeding operations to the usefulness 

 of what we have called "selection index numbers" in 

 such work. The idea of such index numbers is to com- 

 bine in a single numerical expression the values of a 

 series of variable characters with regard to all of which 

 the breeder wishes to practise selection at the same time. 

 The analytical expression of this idea is discussed and 

 its adaptability and usefulness are illustrated by ex- 

 amples drawn from poultry and maize breeding. It is 

 shown that selection index numbers form a valuable 

 adjunct to the score card in stock judging. 



