_Baiomys musculus handleyi_ 617 _Baiomys musculus infernatis_ 618 _Baiomys musculus musculus_ 620 _Baiomys musculus nigrescens_ 623 _Baiomys musculus pallidus_ 625 _Baiomys musculus pullus_ 628 _Baiomys taylori_ 630 _Baiomys taylori allex_ 633 _Baiomys taylori analogous_ 637 _Baiomys taylori ater_ 640 _Baiomys taylori canutus_ 643 _Baiomys taylori fuliginatus_ 645 _Baiomys taylori paulus_ 647 _Baiomys taylori subater_ 650 _Baiomys taylori taylori_ 651 Evolution and Speciation 655 Formation of the Recent Species 658 Areas of present differentiation 661 Zoogeographic position 661 Conclusions 664 Literature Cited 665 INTRODUCTION Pygmy mice (_Genus Baiomys_) are the smallest cricetine rodents in North America. They occur from Nicaragua in Central America into the southwestern United States. The principal part of the geographic range of the pygmy mice lies in the Republic of Mexico. They are notably common in central Mexico, but are only locally common to the north and to the south, and then only in certain seasons.