No. 511] SELECTION INDEX NUMBERS 



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What may be called the final selection index may be 

 formed by combining mathematically the fundamental 

 and special indices in a single index number. 



III. A Selection Index for Poultry 

 The writers are engaged in the experimental breeding 

 of poultry and of maize with reference to utility char- 

 acters. One of the special aims of the poultry breeding 

 work is to learn how to fix superior egg production in a 

 st rain by breeding. In the corn breeding the chief special 

 aim is to improve a strain of sweet corn in respect to 

 earliness, so that it may mature seed under Maine cli- 

 matic conditions. In connection with this work the idea 

 of selection index numbers has been developed. It is 

 proposed to discuss here the particular selection indices 

 which we are using for poultry and for sweet corn simply 

 as an illustration of the theory of - such index numbers 

 and of their application in actual practical breeding 

 operations. It is not our intention to maintain in the 

 least that the indices here given are the best which could 

 be devised or even that they are not subject to change in 

 our own hands with further experience. They are still 

 in the experimental stage. Our only reason for calling 

 attention to the particular forms of indices which we are 

 using at this time is that we believe that the general idea 

 of selection indices as set forth in this paper may prove 

 to be a useful one to the breeder working in other fields. 

 The fundamental idea of a selection index number is a 

 most elastic one. Any one may adopt and modify these 

 indices and weight the different variables, in any way 

 to suit his individual needs. But there is no doubt that a 

 general plan of this kind becomes more intelligible if 

 concrete illustrations of its operation are given. The 

 specific selection indices now to be discussed are pre- 

 sented solely as illustrations of the general theory. 



In the case of poultry breeding a fundamentally de- 

 sirable thing, whatever may be the special object of the 



