30 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



influences resulting in the slight* differences in t he color and texture of the 

 pelage. The northward emigration from this point assumed, probably through 

 the influence of thermometric conditions, the longer fur and especially the 

 dusky tail of the now N. u cinerea"; this attaining its maximum in the Arctic 

 regions. Secondary, slighter differentiations of this branch, due to hygro- 

 metric conditions, resulted in the light-colored variety from the dry plains 

 ("cinerea" of Baird), and the dark variety from the wet, wooded region 

 of Oregon and Washington Territories; in the Arctic regions the color being 

 intermediate, though the pelage is at an extreme of length and density as 

 a protection from cold. Other westward migrants from the Kansas and 

 Colorado center, encountered in the vicinity of Fort Tejon influences that 

 developed N. fuscipes. What these were, we have no idea ; but they are obvi- 

 ously the same that there remodeled Hesperomys leucopus into H. californicus, 

 and made Ochetocbn longicauda out of O. humilis. The southward emigra- 

 tion from the same center, like the northward one, gives results in perfect 

 accord with established and recognized laws. All are smaller, yet with 

 increase in the relative size of peripheral parts ; the comparative dimensions 

 of the tail, feet, and ears being greater. In the New Mexican and Arizonan 

 deserts, warmth and dryness effected the much paler and slightly smaller 

 variety known as N. mexicana ; while still farther south, the greater heat, 

 with less dryness, culminated in the much smaller and very much richer-col- 

 ored N. ferruginea. In this enumeration, we omit N. "micropus", which rests 

 upon intangible characters, and these even not shown to be permanent. 

 However sound this theory may be, or the reverse, it will, we think, be 

 admitted that it explains every phase Neotoma has assumed since the days 

 of N. magister, in strict accordance with laws of geograpbical variation, the 

 general applicability of which no one can intelligently question. 



We have only to add a word here, since we can explain our usage of specific 

 names very perfectly with this case in illustration. So far from believing that 

 any of our Neotomas represent different "species" in the former acceptation 

 of the term, we hold that they are all one, bearing to each other the simple 

 relation of parent and offspring. But we write Neotoma jloridana, N. fuscipes, 

 N. cinerea, and N. ferruginea, because these words severally express definite, 

 tangible, and (comparatively speaking) constant characters which have been 

 impressed upon the parent stem by the physical influences above mentioned 



* Cf. out remarks upon tho difficulty of distinguishing some, examples of N. cinerea and Jloridana 

 from this region. 



