BY RAYMOND PEARL 33 



"normal" mean winter production of Barred Plymouth Rocks (the breed 

 used in this work) is fairly indicated by the 8-year average of the Maine 

 Station flock. This average November 1 to March 1 production is 36 .12 

 eggs. b This figure is based on eight years continuous trap-nesting of the 

 flock with which the present work was done, carried out before these 

 investigations were begun. 



In the second place it is desirable to call attention to some of the difficul- 

 ties which attend an attempt to analyze the inheritance of the character egg 

 production. The most important of these is the fact that this character 

 is not visibly or somatically expressed in the male. A male bird may carry 

 the genes of high fecundity, but the only way to tell whether or not this is so 

 is to breed and rear daughters from him. All Mendelian workers will agree 

 that it is sometimes difficult enough to unravel gametic complexities in the 

 case of characters expressed somatically. It is vastly more difficult when 

 only one sex visibly bears the character. In the second place a very 

 considerable practical difficulty arises from the fact that egg product&n is 

 influenced markedly by a whole series of environmental circumstances, 

 The greatest of care is always necessary, if one is to get reliable results, to* 

 insure that all birds shall be kept under uniform and good conditions,. 

 Further, on this account, it is necessary to deal with relatively large num- 

 bers of birds. Some of the important conditions to be observed in work 

 on fecundity have been discussed elsewhere 6 and need not be repeated here. 



Turning now to the results we may consider first 



THE EFFECT OF SELECTION FOR FECUNDITY IN THE GENERAL POPULATION 

 On the "statistico-ancestral" view r of inheritance it would be expected 

 that if fecundity were inherited at all this character would respond to con- 

 tinued selection. That is, it would be expected, if the highest layers only 

 were bred from in each generation, that the general flock average would 

 steadily, if perhaps slowly, increase and that any level reached would be 

 at least maintained by continued selection. In 1898 an experiment in 

 selecting for high egg production was begun at the Maine station. In this 

 experiment only such females were used as breeders as had laid over 150 

 eggs in their pullet year (corresponding roughly to an average winter pro- 

 duction of 45 or more eggs) and the only males used were such as were out 



6 It should be said that up to and including the winter of 1907 only the November I 

 to March i records are available as a "winter" record. Since that time the small 

 number of eggs laid before November I (on the average two or three per bird) are 

 included in the "winter" totals. These, then, give, as stated, the total production up 

 to March i. 



e Me. Agr. Exp. Sta. Ann. Rept. for 1910, p. 100. 



