No. 5SG] 



SELECTION OF A CHARACTER 



as time has gone on. Nothing of the sort was, in fact, 

 the case. The only explanation which can satisfy the 

 case is one which is based upon or at least takes full 

 account of the changing genetic constitution of the flock. 



It appears to the writer that the essential key-note to 

 the explanation of the results of this long experiment 

 is found in the fact that plucnot ypic variation of the char- 

 acter fecundity, in fowls, markedly transcends, in extent 

 and degree, genotypic variation. It is quite impossible 

 in the great majority of cases to determine with preci- 

 sion what is a hen's genetic constitution with respect to 

 fecundity from an examination of her egg record alone. 

 In this case, as in so many others, but in an unusually 

 pronounced degree, where the pha?notypie distributions 

 overlap, a sure diagnosis of genetic constitution can only 

 be made by means of the progeny test. Lacking this the 

 phaenotypic performance becomes an always uncertain 

 and at times very misleading guide. 



It can be shown that if, during the period of mass se- 

 lection, all the hens used as breeders had been, as they 

 were supposed in the theory of the originators of that 

 part of the experiment to be, either Type 1 or Type 2 

 females (fL,L 2 ■ FU X or fL t L 2 ■ FIX,) then the continued 

 mass selection must have resulted in improvement. The 

 only criterion of constitution which was used, however, 

 was the bird's performance. But, taken by this criterion 

 alone, there would be constantly chosen a proportion of 

 birds whose genotype was for mediocre fecundity, but 

 which made a performance record (phaenotypic) suffi- 

 ciently high to be selected. That this is what actually 

 happened is evident from the curve (Fig. 1), but the fact 

 was experimentally proved in 1909. 8 



Put in the fewest words, then, the reason why no effect 

 was produced during the first ten years of selection and 

 a marked effect was produced during the last seven, was, 

 in the writer's opinion, because genotypically high pro- 

 ducers were uniformly selected (in the high lines) during 



