x EDITORS' NOTE 



who has had much to do with the development of research in these 

 fields. 



Translations from Russian into English are difficult on account 

 of the great differences in habit of thought and action of the two 

 peoples. The style, arrangement, and character of the original 

 Russian have been preserved in this translation in so far as possible 

 for clearness. The style of the English has been modified therein. 

 All corrections have been inserted from the second Russian edition, 

 which is soon to be published. Dr. Maximo v and Mme. T. 

 Krassnoselsky-Maximov have read the edited copy. 



University of Missouri. A. E. MuRNEEK. 



University of Minnesota. R. B. Harvey. 



September, 1930. 



