No. 511] SELECTION INDEX NUMBERS 397 



smoothing effect of combining the four variables into one 

 index number appears most clearly from the diagram. 



4. In Table I, one half of the birds have a selection 

 index equal to or greater than 10. This is about the 

 value which it would seem desirable to take as a divi- 

 sion point in selecting breeders. A bird with an index 

 below 10 can not be regarded as a good breeder. 



Summarizing, we believe the poultry selection index 

 described, or some modification of it to suit particular 

 needs, to be a useful aid in practical breeding operations 

 with poultry. It measures in an exact and impartial 

 manner the performance of a bird as a breeder in any 

 given season. On the basis of the knowledge so gained 

 the breeder can select birds which are to be retained for 

 further breeding. It substitutes an exact and impartial 

 measure, in the place of a rough, general impression of 

 the relative effect of several variables. 



IV. A Selection Index for Sweet Coen 

 The idea of selection index numbers is being applied in 

 connection with investigations in breeding sweet corn 

 (Zea Mays saccharata). In this work the selection 

 index is based on the characters of the ear. A brief dis- 

 cussion of this index is introduced here for two reasons, 

 viz., (a) to illustrate the adaptability of the selection in- 

 dex idea to widely different classes of breeding problems, 

 and (b) to show how such index numbers supplement the 

 score card in breeding operations. In planning this corn 

 selection index the underlying idea was that set forth by 

 way of example earlier in the paper (p. 386, supra). 



The actual index number used for sweet corn has the 

 following formula: 



j A + 3B + 2C 

 i »— D + E + F* 

 The meaning of the letters in the equation are shown in 

 the following scheme. 



A = Length of ear multiplied by the circumference of the 

 ear (L X Cir.). This gives a measure of the ab- 

 solute size of the ear. 



