78 
JOUKNAL OF MAMMALOGY 
I have already reported® that these differences appear to be heredi- 
tary. Unfortunately, only three specimens of the peninsula stock 
have been reared at La Jolla, but all of these are paler than the palest 
of the redwood stock, reared at the same time, and under identical 
conditions. The mean percentage of black in the three skins is 88.3. 
This case may be seized upon in support of the protective colora- 
tion hypothesis, but I am still disposed to adopt the view expressed 
in the paper cited (1917, p. 180): ‘L ... it seems more Kkely 
that the pale coloration of these mice stands in some more direct rela- 
tion to the humidity of their immediate surroundings.’’ Referring to 
the latter, I stated (p. 179) : “Despite the nearness to the ocean and the 
high atmospheric humidity, the peninsula region seems dry in com- 
parison with the redwood forests. This is due in part to the loose, 
sandy character of the soil — where, indeed, any real soil exists — and to 
the comparative lack of shelter from the prevailing westerly winds. 
Evaporation here is doubtless more rapid than in the comparatively 
stagnant air of the forests.” I might have added that the humidity 
of their subterranean abodes, in which these mice are reared and spend 
the greater part of their existence, is almost certainly lower in the sandy 
region. 
The case which has chiefly prompted the publication of the present 
paper is that of the alleged effect of black lava in darkening the pelage 
of certain rodents and other animals which make it their habitat. 
In his well-known “Results of a Biological Survey of the San Fran- 
cisco Mountain Region and Desert of the Little Colorado, Arizona,”^ 
Dr. C. Hart Merriam mentions four different animals which were cap- 
tured in the lava fields of this district, and which differed strikingly 
in color from their nearest relatives in the neighboring desert regions. 
These animals are (using revised nomenclature) : 
Citellus spilosoma obsidianus 
Onychomys leucogaster fuflginosus 
Perognathus flavus fuhginosus 
Phrynosoma hernandesi 
The first, a squirrel, was described as a new subspecies, the second 
(a true mouse) and the third (a pocket mouse) as new species. The 
last (a “horned toad”) is referred to by Doctor Stejneger, who reported 
® American Naturalist, March, 1917, June-July, 1918. 
^ U. S. Department of Agriculture. North American Fauna, No. 3. Wash- 
ington, 1890. 
