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+5) 
Ti = e+td+l1 
in which formula: 
I, = selection index number for a particular individual. 
a = percentage of eggs which hatched. 
b = 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. 
= 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 LX VII.—Sevection Inpices For Barrep PuymMouts Rock Puuuets (Data of 
Pearl and Surface) 
Rand | a | b | c | d qh | Band a b c d qh 
No. No. 
10 | 21) 18.33 |36.0 | 33.33 | 2.8 29 28 | 28.33 | 27.0 0 10.0 
160 | 9 | 15.83 | 42.0 | 0) 2.9 | 23 39 | 35.83 | 28.0 8.33 | 10.0 
402 | 14 | 33.33 | 30.0 50.00 | 2.9 || 428 46 | 40.08 | 20.0 | 22.22 10.0 
352 | 12 | 41.67 14.0 | 60.00 | 3.6 122 49 | 34.17 |15.0 | 23.53 | 10.5 
358 | 50 | 31.67 | 32.0 | 69.23 | 4.0 || 375 41 | 37.50 | 36.0 0 10.6 
438 | 35 | 35.00 45.0 | 37.50 | 4.2 712 42 | 50.00 | 20.0 | 20.00 | 11.2 
441 | 38 | 20.00 133.0 33.33 | 4.3 || 408 48 | 45.86 |16.0 | 22.73 | 11.8 
21 | 25 | 35.00 | 14.0 | 44.44 | 5.0 || 38 | 61 | 37.50 | 9.0 | 28.00 | 13.0 
393 | 12 | 47.50 | 9.0 | 50.00 | 5.0 731 27 | 35.83 | 23.0 0 13.1 
705 | 38 | 26.67 | 34.0 | 25.00 | 5.4 || 395 | 29 | 37.50 | 24.0 0 13.3 
717 | 24 | 21.67 | 19.0 | 20.00 | 5.7 443 | 56) 54.17 6.0 | 35.29 | 13.3 
39 | 23 | 29.17 | 26.0 16.67 | 6.0 || 409 | 37 | 46.67 | 4.0 | 25.00 | 13.9 
377 | 32 | 37.50 | 16.0 | 33.33 | 6.9 | 771 | 43 | 60.00 | 13.0 | 22.22 | 14.2 
746 59 | 28.33 |15.0 | 47.06 | 6.9 19 | 68 24.17 | 24.0 6.66 | 14.5 
87 | 36 | 39.17 | 17.0 | BOT Le She: iele2 | 26 | 39.17 | 11.0 9.09 | 15.5 
359 61 | 32.50 | 15.0) 50.00 | 7.1 366 | 52 | 25.83 |13.0 7.14 | 18.4 
442 | 41 | 68.33 | 38.0 | 28.57 | 8.2 768 74 | 45.00 | 20.0 9.38 | 19.6 
400 | 44 | 47.50 | 16.0 | 38.10 | 8.2 434 72 | 58.33 |17.0 | 14.28 | 20.2 
27 | 18 | 40.00 | 8.3 | 25.00 | 8.5 || 750 57 | 52.50 | 16.0 | 10.00 | 20.3 
757 | 23 | 51.67 | 10.0 | 30.77 | 8.9 || 770 71 | 50.83 9.8 | 12.82 | 25.8 
725 | 33 | 29.17 6.0 | 27.27 | 9.1 | 752 48 | 59.17 6.0 | 12.50 | 27.5 
112 | 17 | 46.67 |18.0 | 25.00 | 9.3 || 397 38 | 40.00 6.0 5.89 | 30.3 
753 | 61 | 47.50 | 46.0 | 10.53 | 9.4 168 88 | 35.83 4.7 | 13.89 | 31.6 
407 41 | 41.67 | 18.0 | 23.53 | 9.7 749 57 46.50 4.0 6.45 | 45.2 
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