142 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
EVOTOMYS RUTILUS GAPPERI, (Vig.) Coues. 
Red-backed Mouse. 
Arvicola gapperi, ViGors, Zool. Journ. v, 1630, 204, pl. 9 (Canada).—DrKay, N. Y. Zool., i, 1842, 91.— 
Scurnz, Synop. Mam. ii, 1845, 252.—ALLEN, Bull. Mus. Comp. Zool. i, 1869, 231. 
Arvicola (Hypudeus) gapperi, Barrp, M. N. A. 1857, 518. 
Evotomys rutilus gapperi, COUES, Proc. Acad. Nat. Sci. Phila. 1874, 187. 
Arvicola fulva, AUD. & Bacu., Journ. Acad. Nat. Sci. Phila. viii, 1842, pt. ii, 295 (name pre-occupied and 
afterward changed to “ dekayi”).—WaGNER, Wiegmann’s Archiv, 1843, pt. ii, 53. 
Arvicola dekayi, AuD. & Bacu., Q. N. A. iii, 1854, 287 (same as their fulva of 1842), (exclude the synonym 
“oneida De Kay”, which belongs to Arvicola riparius). 
Diaenosis.—A. rutilo simillimus, sed statura paululum major, coloribus 
obscurioribus, cauda, pedibus auriculisque longioribus. 
Hasirat.—The northern frontier of the United States, from Atlantic to 
Pacific, and an adjoining belt of British America; further north replaced by 
the true rutilus. Nova Scotia. South to Massachusetts. 
The occurrence of the true A. ruti/us in North America was not more 
unexpected to us than the relationship of A. gapperi, which we had always 
unquestioningly taken as a valid species, proved to be. The differences, as 
detailed by Baird (op. cit. 521), seemed perfectly satisfactory; but, with more 
extensive material than that writer enjoyed, we are enabled to make out a 
different state of the case. Our views would have received strong corrobo- 
ration upon @ priori considerations from the mere circumstance of finding 
the true rutilus in America; but, independently of this, we are prepared to 
present a chain of evidence that cannot be broken down, from direct com- 
parison of specimens irrespective of locality. 
The general tendency of animals that range from temperate to frigid 
regions, to shorten their members, or, as it were, withdraw peripheral parts 
from the cold, and to put on thicker, warmer covering, in higher latitudes, is 
a well-known law, of which the present case merely affords another example. 
We have already seen the condition of the pelage, and the proportions 
of the tail, ears, and feet, in true rutilus, from high latitudes; and the present 
variety, with the same general characters as ru¢édus, differs in the length of 
its several members, and their amount of hairiness, as well as in the condi- 
tion of the general pelage. The difference, however, is not abrupt, nor is it, 
in fact, even well marked, except in its extremes. The transition from typi- 
cal rutilus to the extreme of southern gapperi is so gradual and insensible 
that there is no break in the series. This will be evident from the table 
given below, in which the various examples of gapperi differ as much among 
themselves as some of them do from true ruéédus. Still, comparison of the 
