SUMNER — DESERT AND LAVA-DWELLING MICE 
77 
grass, but a seal brown mouse would probably be no better concealed 
in the former than in the latter. 
We come now to cases in which narrowly localized races have been 
described, that are said to harmonize more or less strikingly with 
some special habitat, in respect to color-tone, and to differ noticeably 
from their near kin in regions closely adjacent. In some of these 
cases, at least, the argument that the responsible agent has been opti- 
cal, rather than atmospheric, assumes a higher degree of plausibility. 
Several writers have recorded the existence of very pale races of wild 
mice upon isolated beaches or sandy islands along the coast. ^ In a 
previous paper,^ I have analyzed one of these cases rather carefully. 
I there dealt with a paler sub-race of Peromyscus maniculatus ruhidus, 
inhabiting a practically isolated sandy peninsula on the northern 
California coast. Through the use of an accurate method of color 
determination,^ I am now able to indicate the relative shades of the 
skins from the peninsula and from the redwood forests of the mainland 
across Humboldt Bay. The proportion of black in the two cases is 
89.0 per cent for the former and 90.9 for the latter. These figures are 
based upon 21 and 29 mature skins respectively. The difference 
(1.9 per cent) is not great, but it is about 12 times its probable error 
(±0.16) and therefore cannot be accidental. Furthermore, it is to be 
remarked that all but one of the peninsula mice show less than 90 per 
cent of black; while all but three of the redwood mice show more than 
90 per cent. Indeed, the difference between the two series is quite 
evident to the eye, even upon casual inspection.^ 
2 For instance, Bangs (see Osgood, North American Fauna, No. 28, 1909, p. 
121) described such a race from the island of Monomoy on the Massachusetts 
coast. See, also, interesting recent accounts by G. M. Allen and by A. H. Howell 
(Journ. Mamm., November, 1920). 
2 American Naturalist, March, 1917; Osgood (Revision of the Genus Pero- 
myscus, p. 66) had already referred to the case in question. 
^ Flat skins, prepared according to a uniform method, and cleaned in benzine, 
are subjected to color analysis by means of the Hess-Ives Tint Photometer. In 
the present paper, the figures given represent the average tone of an area of the 
pelage, 24 mm. wide by 17 long (the dimensions of the visible field) and lying sym- 
metrically, near the posterior end of the dorsal surface. One great advantage 
of the instrument used is that the area under analysis is rendered perfectly homo- 
geneous and is thus strictly comparable with another homogeneous field which 
serves as the standard. Later I hope to discuss the use of this instrument in 
the study of mamnialian pelages. 
® If we consider the percentages of white, rather than of black, we have 9.2 
and 8.0 respectively for the two series. This makes the difference 15 per cent of 
the lesser number, A much smaller increase in the proportion of white upon a 
color-wheel is very evident to the eye. 
