CHECKLIST OF THE SNAKES OF MEXICO 11 



not unreasonable. It is well to bear in mind, in this connection, that 

 snakes are apparently not a decadent but definitely an expanding 

 group. It is clear that the oldest ("South American") paleofauna is 

 the one most involved in the resurgence of southern groups. The "Old 

 Northern" is involved chiefly in the essentially Nearctic group of 

 genera, while the newer "Holarctic" is not at all involved in Mexican 

 snakes. We do not believe it feasible at the present time, how^ever, to 

 attempt an exact identification of all the snake genera of Mexico as 

 "South American" or "Old Northern." 



Had the "South American" and "Old Northern" faunas retained 

 their distributional relationship even in recent periods, the realm now 

 distinguished as Neotropical would be composed exclusively of deriva- 

 tives of "South American" elements, and the Nearctic would consist 

 of two subdivisions corresponding to the "Old Northern" and "Hol- 

 arctic" elements. The establishment of secondary centers of dispersal, 

 however, resulted in such an overlap of the derivatives of three paleo- 

 f aunas that the zoogeographic realms we now recognize as correspond- 

 ing with present distributional facts unfortunately have little correla- 

 tion with paleozoogeography. 



ACKNOWLEDGMENTS 



Many friends and colleagues have been of very material aid in the 

 construction and completion of this project. In particular we wish 

 to acknowledge the careful reading of most of the manuscript by Dr. 

 Emmett Reid Dunn, Dr. Norman E. Hartweg, Karl P. Schmidt, and 

 Dr. L. C. Stuart, whose intimate knowledge of the Mexican and 

 adjacent faunas has made them valued critics. We are also indebted 

 to Dr. Hartweg for permission to include data on his recent collections 

 that are undergoing study at the present time, and to Dr. Dunn for the 

 answers to numerous queries, the loan of specimens, and several useful 

 suggestions. Among others to whom we are deeply indebted, for 

 loan of specimens, or for suggestions, or who have otherwise aided 

 materially, are Dr. Joseph Bailey, Dr. Thomas Barbour, Dr. S. C. 

 Bishop, C. M. Bogert, Dr. Doris M. Cochran, Eoger Conant, Miss 

 Anita Daugherty, Mrs. Helen T. Gaige, Dr. Howard K. Gloyd, Dr. 

 Arnold Grobman, L. M. Klauber, Arthur Loveridge, Dr. James A. 

 Oliver, Clifford H. Pope, Thomas Rodgers, Benjamin Shreve, Dr. 

 Joseph R. Slevin, William Stickel, Dr. Leonhard Stejneger, Wilmer 

 Tanner, Dr. Vasco M. Tanner, Joseph Tihen, Dr. Alexander Wetmore, 

 and Dr. A. M. Woodbury. Finally, we are indebted to Paul H. Oehser, 

 editor of the United States National Museum, for his thorough and 

 sympathetic completion of the arduous task of editing and supervising 

 the publication of the manuscript. 



