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THE AMERICAN NATURALIST [Vol. XLIX 



mean and median do not coincide. In Table II is shown 

 the percentage of the whole flock laying 18 or more eggs 

 in the months specified. 



This table shows in an even more striking way than 

 the means in Table I the marked difference between the 

 flocks of the present time and those of the earlier years. 

 In 1913-14 nearly half the flock laid 18 or more eggs each 

 during December and January. 



The data presented in this paper establish, I think, 

 the following facts : 



1. There is a marked difference in the average produc- 

 tion per bird of Barred Plymouth Rock pullets of the 

 Maine Station strain at the present time, as compared 

 with what obtained in the earlier trap-nesting work of the 

 Station described by Pearl and Surface (loc. cit.). 



2. This difference is in the direction of a substantially 

 higher mean flock production at the present time. 



3. The increase in flock production is most pronounced 

 in respect to winter production. 



The most probable explanation of the above results 

 appears to the writer to be that the plan of breeding now 

 followed is more nearly in accord with the biological facts 

 regarding the inheritance of fecundity than was the plan 

 followed in the earlier years. 



The reasons for this opinion, while not constituting 

 complete proof of the suggested explanation, certainly 

 make a strong body of evidence in its favor. They are, 

 summarily stated: 



(a) That the increases in flock productivity have been 

 synchronous with changes in breeding practise. 



(b) That the increases give every indication of being 

 permanent, there having been no tendency towards a de- 

 cline in flock productivity since 1908, when the simple 

 mass selection was stopped and breeding begun on a 

 progeny-test basis. 



(c) That there have been no changes in management or 

 environmental circumstances synchronous with the in- 

 creases in flock production and capable of accounting for 

 them. The hens are housed to-day in the same houses 



