176 RAYMOND PEARL 



pedigree bred for a number of years and their hereditary quahfi- 

 cations in respect of egg production were well known (Pearl, 18). 

 I have lately called attention (23, p. 134) to some of the diffi- 

 culties which are involved in interpreting critically and fairly 

 the results of physiological experiments in which egg production 

 is used as an indicator. One must have proper controls in the 

 first place. The ideal condition in respect of controls is only 

 reached when one has full sisters of the experunentally treated 

 birds. This condition has been met in the present experiments. 

 In the second place the management of the birds in the experi- 

 ment should be such that one can be sure that the control birds 

 are laying normally throughout the experiment. By normal 

 laying in this connection is meant the full somatic expression of 

 the innate inherited capacity of the buds for egg production. 

 Unless this is reached by the controls in any physiological 

 experiment on egg production one never can be quite sure that 

 any difference which may appear between the control and 

 experimentally treated birds is not due to the effect of some 

 overlooked environmental factor upon the controls, which re- 

 duced their production below what it should have been. 



The egg production of all the birds in these experiments, by 

 months, is given in table 4. In this table 'D' means that the 

 bird in question died in the indicated month after laying the 

 number of eggs in that month shown by the figures preceding 

 theD. 



From this table it is evident that all of the birds, both treated 

 and untreated, have laid normally throughout the experiment. 

 To get beyond this general impression and make exact compari- 

 sons between controls and treated birds it is necessary to reduce 

 the mass of figures of table 4 to means or averages. This has 

 been done in table 5. In calculating the means in table 5 only 

 such birds have been included as began their first laying year 

 at the beginning of the experiment, namely the autumn of 1914. 

 Those birds which were yearling hens at that time (treated 

 birds numbered in the 1500's and theu* sisters) were entering 

 their second laying year. First yeapr and second year laying 

 records are not homogeneously comparable, and therefore should 



