560 GENETICS IN RELATION TO AGRICULTURE 
domestic fowl. The conclusions are based upon an analysis of data 
obtained by trapnesting strains of pure-bred Barred Plymouth Rocks 
and Cornish Indian Games, and F, individuals and F individuals ob- 
tained by mating F; individuals inter se and by mating them back to 
their parents in all possible combinations. Over a thousand birds were 
subjected to this definite experimental test. 
With respect to winter egg production hens naturally appear to fall 
into three well-defined classes; (a) those birds which lay no eggs during 
the winter period; (b) those which lay something less than about thirty 
eggs; and finally (c) those which lay more than thirty eggs. Since egg 
laying is a character strongly influenced by environmental conditions 
120 
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: A 
60 
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ee 
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0 Set 
90 80 710 60 50 40 30 20 10 Q 
Per Cent Producing Indicated Number of Eggs or More 
Fic. 220.—Contrasted flock curves of winter egg production of Barred Plymouth Rock 
(solid line) and Cornish Indian Game (broken line) pullets. (After Pearl.) 
and somatic fluctuations, these classes are not absolute, nor on the other 
hand are they by any means purely arbitrary as has been determined 
by statistical studies of flock production during the winter period. The 
differences which exist between the two breeds under investigation are 
shown graphically in Fig. 220. Taking a production of thirty eggs or 
more as the standard of comparison between the two breeds, it may be 
seen from this graph that only about 6 per cent. of the total flock of 
Cornish Indian Games produced as many eggs as this during the winter 
period, whereas 54 per cent. of Plymouth Rock pullets measured up to 
this standard of excellence. 
From a consideration of the data obtained from a wide series of 
crosses, Pearl proposes the following analysis of the inheritance of 
fecundity in fowls as measured by winter egg production. For the sake 
of clearness and conformity to treatment in the remainder of the text, we 
have used symbols different from those used by Pearl without, however, 
in any way modifying the essential features of his analysis. 
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