504 



GENETICS IN RELATION TO AGRICULTURE 



The second difficulty has to do with determining some method by 

 which selection may be made effective for a number of characters, or 

 in other words a method of comparing different individuals with respect 

 to a number of different characters. The use of selection index numbers 

 provides a way of surmounting this difficulty. Pearl has made use of 

 the following fundamental selection index in the Maine Station poultry 

 work: 



5(a + b) 

 c + d + 1 

 in which formula: 



Ii = selection index number for a particular individual. 



a = percentage of eggs which hatched. 



6 = percentage of eggs actually laid to the total number which might 

 have been laid during the breeding season, February 1 to June 1. 



c = percentage of infertile eggs. 



d = percentage of chicks which died within 3 weeks after hatching. 



The application of this index to different hens in a breeding flock is 

 shown in Table LXVII. The selection index ranges in value from 2.8 to 



TABLE LXVII. SELECTION INDICES FOR BARRED PLYMOUTH ROCK PULLETS (Data of 



Pearl and Surface) 



