Acknowledgments and Foreword 



As in my previous paper on the Crop Centers, I am indebted 

 to Dr. E. N. Transeau for ecological criticism and guidance. The 

 idea of centers of plant and animal life about which certain forms 

 are naturally grouped is originally his and Dr. C. C. Adams's. Its 

 chief advantage, as opposed to the temperature or other "zone" 

 plan of mapping distribution is that it works. It should be clearly 

 borne in mind by the reader that boundary lines as drawn do not 

 attempt to mark divisions in any absolute way, but do show the 

 centers of distribution. For definitions of the biotic center 

 Adams's and Transeau's papers should be consulted. The writer 

 is also indebted to numerous other friends who have aided him in 

 one way or another, and most especially to those who corresponded 

 with him shortly after the publication of the Crop Centers paper. 



Due to unavoidable delays in the University publications, the 

 date of the actual completion of the manuscript and the date of 

 going to press shows an interim of two years. In the meantime 

 a number of valuable papers of an ecological nature have appeared, 

 but a discussion of their bearing on the field of crop ecology must 

 be reserved for future work. 



A. E. W. 



August 24, 1920. 



