VARIATION IN DOMESTIC ANIMALS 459 
Modifiability and Correlation.—Low correlation does not necessarily 
indicate the relative degree and importance of modifiability in a popula- 
tion, for it is possible to have low correlation between parent and off- 
spring as a consequence of genetic variability. This matter may be 
pointed our very clearly by considering a few abstract cases. Thus a 
population consisting largely of different, but homozygous, forms would 
exhibit a high degree of correlation between parent and offspring; 
whereas one containing more heterozygous individuals would display 
a lower correlation coefficient, simply because of the segregation which 
would take place in sucha population. This, of course, is simply a de- 
velopment of the general case that an individual of genotype AA would 
produce only A individuals, provided A were completely dominant, 
whereas the genotype Aa would produce individuals of its own type and 
also of type a. On the other hand, modifiability may be the factor 
determining the value of the coefficient of correlation, even when the 
degree of conformance to a given standard is very high. Thus if a 
population be homogeneous genetically, as is the case in pure lines, then 
the correlation coefficient within the population would be nil, but when 
a number of different pure lines are mixed together and the correlation 
coefficient is determined for the mixed population, the value would be 
very high, although each pure line within itself would exhibit zero 
correlation. 
This matter requires specific attention because some little confusion 
has grown up from the use of the coefficient of correlation as a measure of 
the intensity of inheritance, a practice of doubtful scientific propriety 
and one which might well favor misleading conclusions. The lack of 
correlation, for instance, in egg production of hens and their daughters 
theoretically does not indicate that attempts to increase egg production 
will be absolutely futile. It does indicate that some method of 
breeding must be adopted that will discount at their proper value the 
influence both of modifiability and genetic variability consequent upon 
segregation. The theoretical interpretation in this case is borne out 
strikingly by the practical results of the application of the method of 
genotypic selection, for as is shown graphically in Fig. 185, there has 
been a striking increase in egg production from the year in which the 
method of breeding was changed. 
Variation by: Recombination.—Unquestionably the greatest possi- 
bility for improvement in animal breeding as well as in plant breeding 
lies in the isolation of recombinations of germinal elements which are 
better adapted to specific purposes. There is every reason to believe, 
as we shall point out in the next following chapter, that the architecture 
of the germ-plasm, if we may use such a term, in domestic animals is 
similar to that in Drosophila, that is, that the elements of the germ-plasm 
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