52 BULLETIN 1339, U. S. DEPARTMENT OF AGRICULTURE 



(34) Wright, Sewall. 



1921. Correlation and causation. In Jour. Agr. Research, vol. 20, 

 No. 7, pp. 557-585, 16 figs. 



(35) 



1923. The theory of path coefficients. A reply to Niles's criticism. 

 In Genetics, vol. 8, pp. 239-255, 8 figs. 



(36) Yule, G. Udny. 



1919. An introduction to the theory of statistics, xv, 398 pp., 53 

 figs.. London. 



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