168 



TECHNICAL BULLETIN 8 



The Coefjicievt of Correlation. 



A general survey of a series of individual egg records may lead to cerlain 

 general deductions some of wliicli may actually lie true and others false. The 

 breeder needs to know just how uuicli stress to lay upon dift'erent character- 

 istics associated with the traits concerned in high fecundity. The simple 

 correlation coefficient affords a concise measure of the degree of association 

 between specific traits and high fecundity as well as a measure of the rela- 

 tionship between the presence of particular traits and high fecundity. The 

 correlation coefficient may thus be made use of by the breeder in two ways: 

 first, for prediction purposes; and second, in the selection of breeding stock 

 to obtain 'the most valuable conUunation of traits. Stated concisely, tlie cor- 

 relation coefficient is the only direct and specific measure for degree of 

 association of characteristics wliere large nnmliers of indiN iduals are con- 

 cerned. Fecundity records may he modified l)y a vast number of environ- 

 mental conditions as well as l)y the five traits pointed out by Goodale and 

 Sanborn (1922) which are shown to be inherited. Hays (1924.). Only by 

 the use of large numbers of records made over a period of years under uni- 

 form conditions of management and in a ilock bred for uniformity can a 

 Irue value of the relative importance of characteristics concerned with fecun- 

 dity be approached. The coefficient of correlation tluis becomes an invalu- 

 a!)le tool in breeding for fecundity. 



Pret>enlation of Ihda hif Fatnilies. 



A stud\ undertaken to consider the winter cycle and winter jiause by 

 separate families would necessitate the presentation of page after page of 

 abstract data. Such data should be accompanied by detailed and complete 

 discussions and such general deductions as would seem justified. No definite 

 constants could be deternnned on numbers so small as the individual family. 

 Tossibly all the descendants of particular individiuils could be considered 

 as unit.s, but from the genetic stand))oint such a consideration should be 

 classed as questionable. A general tabulation of the whole i)opulaiion, 

 giving such information as mean hatching date, mean age at first egg, mean 

 weight at first egg, percentage of birds pausing, mean length of pause, mean 

 winter ])roduction, mean annual production, etc., by years could l)e made. 

 Such a tabulation would again be open to the criticism of not furnishing 

 specific information. Only general deductions could be made and no evi- 

 dence would be furnished as 1o relative values. In view of the above facts, 

 this method of handling the data is not considered feasible. 



Winter Cvci.e 



Winter cycle may be considered in three general categories: namely, (a) 

 in its relation to environmental conditions, (I)) in its relation to heritable 

 characteristics concerned in fecmuhty, and (c) in its absolute relation to egg 

 production. 



(a) Relntion of Environmental Conditions to Winter Cycle. 

 Hatching date belongs to the definitely controllable class of conditions in 

 that it may be varied at will of the investigator. Date of first egg dei)ends 



