Line and grade in the extinct medius species group of sigmodon. 
Remains of Sigmodon minor and Sigmodon medius, extinct cotton rats comprising the medius species group, have been recovered from both Kansas and Arizona.  Sigmodon minor evolved from Sigmodon medius, and the available evidence suggests that Kansas populations of Sigmodon minor were derived from Kansas populations of Sigmodon medius, while Arizona populations of Sigmodon minor were derived from Arizona populations of Sigmodon medius.  This multiple origin of a single mammalian species is similar to the origin proposed by Carleton Coon for the races (subspecies) of Homo sapiens.