PREFACE IX 



domains of chemistry and physics. It is an unpossible 

 task for any one individual to cover thoroughly the litera- 

 ture even in one of these fields of research, and I must beg 

 the indulgence of my critics in this matter. In my desire 

 to submit these studies in a small and concentrated volume, 

 I have passed over or mentioned only briefly many deserving 

 and creditable pieces of investigation. Obviously, some 

 researches of importance may not have come to my notice. 



I have made extensive use of Professor Paul Buchner's 

 monograph "Tier und Pflanze in Intra-zellularer Symbiose." 

 This comprehensive review of the hterature on ''micro- 

 symbiosis" has shortened my labors by many months. The 

 extensive treatise by Professor Edmund B. Wilson in his 

 recent revision of "The Cell in Development and Heredity," 

 Professor E. G. Conklin's lectures on "The Mechanism of 

 Evolution," Professor Vernon L. Kellogg's "Darwinism 

 Today," Professor H. F. Osborn's "The Origin and Evolu- 

 tion of Life," and many other treatises mentioned in the 

 text have served as short-cuts to a better appreciation of 

 the accomphshments in various biological fields. I grate- 

 fully acknowledge my indebtedness to these valuable works. 



To Professor Arthur I. Kendall, Doctor F. Lohnis and 

 Doctor Montrose T. Burrows, I am especially indebted for 

 sympathetic interest and encouragement in my investiga- 

 tions and for many valuable suggestions. To my colleague 

 Doctor Ralph G. Mills I am indebted for painstaking 

 criticisms of the manuscript. I am, also, greatly indebted 

 to other colleagues at the University of Colorado School 

 of Medicine for opportunities to discuss various phases of 

 my investigations and hypotheses. To my technicians, 

 Mr. Ivan M. Way and Aliss Josephine Jones, I am especially 

 grateful for conscientious and rehable assistance in the 

 technical work of the researches. 



It is a great pleasure to acknowledge my indebtedness 

 to Mrs. Henry S. Denison for research funds which were 



