Specimens to the total number of 335 from Kansas have been available for the present study of the five subspecies recognized. The reason for arranging all of the named kinds as subspecies of a single species is that intergradation has been found to occur between every pair of kinds having contiguous geographic ranges. The characters previously thought by some writers constantly to differentiate, say, _Geomys lutescens_ of western Kansas from _Geomys bursarius_ of eastern Kansas, prove not to do so; instead, in areas geographically intermediate between the geographic ranges of the two kinds, the pocket gophers are intermediate in morphological characters and therefore are regarded as intergrades. Intergradation of this kind here is accepted as the criterion of subspecies, and lack of such intergradation as the criterion of species. Search for structural characters, distinctive of the different kinds, additional to those characters noted by other writers, has resulted in the finding of a few such characters but they too are subject to intergradation. Therefore the several kinds are arranged as subspecies of a single species which takes the name _Geomys bursarius_ because it is the oldest available name. Detailed comment on specimens showing intergradation are to be found in the accounts of _G. b. bursarius_ and _G. b. major_.
The series with the largest number of individuals from one restricted locality was selected for initial study. These individuals were segregated by sex, and specimens of each sex were arranged from oldest to youngest. Each series was divided into age-groups, and within a given age-group of one sex from one locality of what was considered as one species, estimation was made of the amount of individual variation. Thus, it was possible when comparing different kinds of pocket gophers to use only one age class of one season of one sex.
Age was estimated to some extent by size of animal and nature of its pelage. The immature pelage is grayer and the hair is more crinkled than in adults. A more certain guide to age, however, is furnished by the skull. With increasing age some sutures disappear, the rostrum increases in length and the ridges marking the limits of the temporal muscles come to fuse and eventually, in males, form a high sagittal crest.