30 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
influences resulting in the slight* differences in the 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 WV. “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 WV. 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 Ochetodon 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 WV. mexicana; while still farther south, the greater heat, 
with less dryness, culminated in the much smaller and very much richer-col- 
ored NV. ferruginea. In this enumeration, we omit LV. “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 Meotoma has assumed since the days 
of NV. magister, in strict accordance with laws of geographical 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 floridana, N. fuscipes, 
N. cinerea, and N. ferruginea, because these words severally express definite, 
tangible, and (comparatively speaking) constant characters which haye been 
impressed upon the parent stem by the physical influences above mentioned 
* Cf. our remarks upon the difficulty of distinguishing some examples of N. cinerea and floridana 
from this region. 
