498 GENETICS IN RELATION TO AGRICULTURE 
iveness of purely phenotypic selection for fecundity in the Barred 
Plymouth Rock is indicated by the plan which was followed during 
this portion of the investigations. Only pullets were used for breeding 
stock which had laid 150 or more eggs during their first laying year, 
and cockerels were selected from among the progeny of 200 egg hens. 
The type of selection practised was, however, strictly mass selection, for 
the selected birds were bred together without respect to genetic relation- 
ship, and no tests were made of the laying capacities of progenies from 
particular matings. This last point is of particular importance, because 
it definitely distinguishes the method of breeding used as one typically 
of mass selection. 
Obviously the reason for the ineffectiveness of selection during this 
period of mass selection lies in the fact that modifiability in fecundity 
is very great. This particular fact has been discussed fully in a pre- 
ceding chapter, but here it must be considered again as the reason for 
the fact that this system of selection failed to result in improvement in 
egg-laying capacity, for were performance and genotypic constitution 
closely correlated, then this system of mass selection should have been 
effective. But as a matter of fact the criterion of selection used in this 
portion of the investigations, namely total yearly egg production, was 
evidently not a good index of genotypic constitution, for apparently 
it failed to distiguish between individuals belonging to a number of 
intergrading genotypes. Consequently, whenever, by chance a female 
was selected which by phenotypic variation represented the upper 
limits of her genotypic class, the population was thereby thrown back 
by that much to the level representing the mean phenotypic performance 
of her particular genotypic class. A wide range of modifiability for each 
genotype, therefore, continually held the average yearly production 
down to the original value for the population. 
But beginning with the year 1908 a radical change was made in the 
method of selection. During the first portion of the second period, 
the object was merely to ascertain the actual mode of inheritance of 
fecundity, a subject which is discussed more fully elsewhere; but during 
the second portion, from 1912 to the present time, selection was only 
carried out for high egg production. Essentially, however, the mode of 
selection during these two portions of the second period was the same so 
that we may consider this as a single period. The performance index 
during this period was winter egg production rather than total egg 
production. But in the selection of high winter producers for breeding 
purposes, a progeny performance test was employed as well as an actual 
individual performance test. Every female which was selected during 
this period came from a high-producing mother, the female progeny of 
which were all high producers, In case such a female failed to give 
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