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THE AMERICAN NATURALIST 



[Vol. XLIX 



production, there ought to result a marked and immediate 

 improvement in average flock production no matter what 

 the size of the flock. 



This, as a matter of fact, is exactly what has been done 

 in the breeding of the flock of Barred Plymouth Rocks at 

 the Maine Station for several years past. No attempt has 

 been made to propagate low fecundity strains, after it had 

 once been demonstrated that this could be done. In the 

 work since 1912 the experimental aims have been such as 

 not to be at variance with the practical one of getting the 

 most eggs with the least trouble and expense, so far as 

 has concerned the Barred Plymouth Rock stock. Conse- 

 quently in making the matings from which the founda- 

 tion Barred Plymouth Rock stock was being maintained 

 I have each year endeavored to keep a number of differ- 

 ent blood lines comparatively pure for the factors L x and 

 L 2 , and then intercross these lines with one another. 



The results have been highly successful from a prac- 

 tical point of view. This is indicated by the figures shown 

 in Table I and graphically in Fig. 1. These compare the 

 mean egg production per bird month by month under the 

 old system of mass-selection and under the new system 

 of breeding which recognizes the Mendelian inheritance 

 of fecundity with sex-linkage of the factor on which high 

 production depends. The figures for the new system arc 

 those of the laying year 1913-14. In the laying year 

 1912-13 the flock had not yet attained any considerable 

 degree of homogeneity in respect of fecundity factors 

 since up to and including the preceding year low produc- 

 ing genetic combinations had been deliberately propa- 

 gated and therefore an average which included all birds 

 in the flock would be manifestly unfair as a test of the 

 practical worth on a large scale of the new systems of 

 breeding. The laying year 1913 14 is then the first com- 

 pleted year on which records are available for a fair test 

 of the Mendelian plan on a total flock scale. 



The Barred Rock flock of the year 1913-14 included 192 

 birds which completed the year's work. A number of 

 other birds (about 20) began the year but died before its 



