BREEDING POULTRY FOR EGG PRODUCTION. I45 



lying principles of the inheritance of fecundity in fowls. One 

 set of facts having been accumulated, efforts were turned 

 towards getting more data of a somewhat different character. 



The keynote to the new line of investigation has been the 

 analysis of the inheritance of fecundity by means of individual 

 pedigrees. By this method one determines precisely the behavior 

 of each individual in inheritance. Those individuals of like 

 hereditary behavior or performance may then be lumped to- 

 gether for statistical treatment if desired. The "individual 

 pedigree" is the nearest approach which can be made in an 

 organism in which each individual is of one sex only to the 

 genealogical unit termed by Johannsen a "pure line" in self-fer- 

 tilizing plants. Its employment in the analysis of inheritance 

 in animals has underlying it the same considerations which make 

 the "pure line" so potent an instrument of research in plants 

 and non-sexually reproducing animals. 



In order that what follows in Part IV may be more readily 

 understood it is desirable here to explain fully the working 

 hypothesis which the present study in the inheritance of fecun- 

 dity is testing. To put the matter most briefly it may be said 

 that this hypothesis is an adaptation to the particular case in 

 hand of the genotype concept of Johannsen. In more detail the 

 case is as follows. 



Johannsen in his work on beans * brought out very clearly 

 three things which in themselves and in their implications are of 

 fundamental importance to all practical breeders of animals or 

 plants, as well as to students of breeding. These three things 

 are: 



I. That the size of an individual bean was no absolute or 

 certain criterion whatever as to the average size of its offspring. 

 He found that while some particular large beans always pro- 

 duced large offspring beans, other equally large ones always 

 produced small offspring beans. Some individual small beans 

 ]jroduced offspring of large average size, others produced beans 

 of small average size like the parent, and, in general, he showed 

 it to be quite impossible for anyone to tell merely from the size 

 of a bean itself whether its progeny will be large or small. 



* Johannsen, W., Ueber Erblichkeit in Populationen und in reinen 

 Linien, Jena, 1903. , 



