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THE AMERICAN NATURALIST 



[Vol. LII 



the appearance of the first egg. While a true mediocre 

 producer such as No. 5080 can easily he distinguished 

 from a high producer in Barred Plymouth Eocks, it is 

 impossible to draw a division line on the basis of the 

 number of eggs produced where the egg production is of 

 the sort observed in our Ehode Island Eeds. (See Figs. 

 3 and 4.) Except in the case of a few individuals, the 

 only evidence for the existence of two genetically distinct 

 groups, such as Pearl found for Barred Plymouth Eocks, 

 is in the shape of the left-hand portion of the curves of 

 winter egg production. If the zero producers be omitted 

 as a wholly artificial group (cf. Figs. 8 and 17) this evi- 

 dence becomes less satisfactory, especially as the excess 

 of numbers on this side of the curve may well be due to 

 environmental factors, since the effect of such factors is 

 almost always in the direction of decreased production. 

 Moreover, in a later paragraph it is shown that the shape 

 of these curves depends upon certain clearly recognized 

 factors. 



In order to eliminate any miscomprehension in regard 

 to the characteristics of a mediocre producer, I have gone 

 over the matter personally with Pearl. It appears that 

 his Barred Plymouth Eocks, with the exception, of course, 

 of the zero producers, begin to lay at about the same age, 

 but lay at widely diff ering rates. My Ehode Island Eeds, 

 on the contrary, begin to lay at widely different ages, but 

 lay at a fairly uniform rate. In the Barred Plymouth 

 Eocks, therefore, there are three distinct types of winter 

 records, zero producers, mediocre producers (Fig. 12, 

 Nos. 274 and 284) and high producers (Fig. 3). The 

 same types of records have appeared in a flock of Brown 

 Leghorns which we have trap-nested. 



In our Ehode Island Eeds, records of the mediocre 

 type are so rare that they must be referred either to some 

 non-genetic origin, or a chance union of two hetero- 

 zygotes, such as results in the appearance of an occasional 

 recessive in a flock that characteristically has some domi- 



