IV PREFACE. 



The author is under deep obligation to Dr. D. T. MacDougal for 

 constant aid and advice in the prosecution of the field work and in the 

 preparation of the manuscript. Grateful acknowledgment is also 

 made of the help given by Dr. Edith Clements, who has assisted 

 throughout the field work, and in preparing the illustrations and in 

 revising the manuscript. Special acknowledgment is due Dr. F. H. 

 Knowlton for his generous aid in making a copy of his forthcommg 

 catalogue of Mesozoic and Cenozoic plants, which has formed the basis 

 for the discussion of plant populations in Chapters XIII and XIV, 

 as well as for his kindness in reading Chapters XII to XIV. The 

 author is also under obligation to Professors Chamberlin and Salis- 

 bury and to Dr. Huntington for the extensive use made of their pub- 

 lished results. Prints for many of the illustrations have been furnished 

 by Professors A. G. Tansley, C. E. Moss, Ove Paulsen, G. E. Nichols, 

 E. H. Barbour, R. J. Pool, and J. E. Weaver, and by Mrs. H. C. Cowles, 

 Dr. C. E. Johnson, Mr. Harvey Stallard, and the United States Forest 

 Service, to whom due acknowledgment is made. 



Frederic E. Clements. 

 University of Minnesota, June 1915. 



