Exploration of the Colorado River of the West and Its Tributaries Explored in 1869, 1870, 1871, and 1872, Under the Direction of the Secretary of the Smithsonian Institution.

Page 220, Exploration of the Colorado River

220 EXPLORATION OF THE CANONS OF THE COLORADO. perfectly well-discriminated species, the early links between which have entirely disappeared, leaving the forms as stable as possible. Thomomys is still in a transition-stage at present. Setting aside the peculiar T. dusius, described as new beyond, all the known forms of that genus are still only incompletely separated, and the links binding them are plainly before our eyes. The genus appears to be making into a number of species, but the process is still far from completion. In talking over this singular difference in the natural history of these two closely-allied genera with Professor Baird, he threw out a suggestion, which, perhaps, may account for the facts. While we have no means of knowing which is the older of the two genera, so as to compare the rates of progress they respectively made in developing their species, yet we may fairly infer, upon geographical considerations, that Geo-mys has been longer about it than Thomomys. Geomys, though found to a certain longitude westward, is essentially a form of Eastern North America, extended thence to Central America, and it is presumably upon older ground than the late deposits in the West, where Thomomys occurs. In the cases of many mammals and birds of this continent, it will be recollected there is a perfectly stable eastern species of a genus which in the West is represented by a number of "varieties.'' Junco, Melospiza, Passerelia, Picus, are good illustrations. But be the explanation what it may, there is no doubt about the fact that Geomys has made itself into five or six firm species before Thomomys has succeeded in turning out more than one or two. With these few observations by way of introducing the species, I shall proceed at once to give an account of them, reserving many details which contributed to the result of my investigation, as well as all general considerations respecting the family, for publication in a different connection. Sj sp. SHAW, et al., 7. c. infra. Cricetus, sp. DESM., et al., I. c. infra. Geomys, RAF., Am. Month. Mag. ii, 1817, 45. Diplostoma, RAF., op. et loc. cit. (char, pleruraq : inept.), nee RICH. SaccophoruSj KUHL, Beitr. 1820, 65. Pseudostoma, SAY, Long's Exp. R. Mt s. i, 1823, 406. Sj LICHT., Abhand. Berl. Acad. 1822-'3, 1825, 20.