I dedicate this work to Don Hunsaker, II, who suggested the initial problem; to R. W. Axtell, who got me off to a good start on it; and to E. E. Williams, who saw it through to at least a partial completion. Without their assistance and moral support throughout, the work would hever have been done.
Ticul Alvarez-S. assisted with all aspects of the work in Mexico. A. Ornstein, S. M. Moody, T. Dickinson, S. Reichlin, R. B. Stamm, R. Garcia-L., A. Ocana, P. Huerta-M., and H. Pedraza-M. all provided extensive help in the field. Valuable specimens were provided by K. Larsen, L. J. Bussjaeger, R. F. Clarke, R. W. Axtell and others. A. Russell and H. F. Londe assisted with the cytology. Drs. R. Hernandez-C. and B. Villa-R., both of the Direccion General de la Fauna Silvestre, SAG, arranged for collecting permits and assisted in other respects. Laboratory space for cytological work in the field was provided by Texas A & I University, AMNH Southwest Research Station, California State College-San Diego, Escuela de Ciencias Biologicas of the Universidad de Nuevo Leon, Escuela Nacional de Ciencias Biologicas of the Institute Politecnico Nacional, and the Departamento de Prehistoria of the Institute Nacional de Antropologia y Historia. The Departamento de Prehistoria also provided maps, surveying equipment, and a jeep for much of the field work in the Valley of Mexico.
Many of the issues in this study were discussed with E. E. Williams, E. Mayr, R. K. Selander, R. W. Axtell, and H. M. Smith. I thank Williams, Mayr, and R. C. Rollins for critically reading the manuscript and making many useful suggestions for improvement. Any faults that remain are mine. I also thank Catherine McGeary for her assistance in preparing the manuscript.
This research was supported by grants from the National Geographic Society Committee on Research and Exploration, the Society of the Sigma Xi (both to Hall), and the National Science Foundation (GB-6944 and B-019801-X to E. E. Williams, Harvard; and GB-3167, 7346, and 19922 to R. C. Rollins, Harvard).