May 24, 1912] 
I am indebted to Professor J. O. Snyder, of 
Stanford University, and to Professor Joseph 
Grinnell, of the Museum of Vertebrate Zool- 
ogy, University of California, for suggestions 
and for the use of the collections under their 
charge. To Miss Hilda Hempl, of Stanford 
University, I am indebted for a considerable 
number of specimens. 
The common house mouse is nearly uniform 
in color all over. The under parts are a little 
lighter than the back, but the transition is 
very gradual. The entire color of the mouse 
is subject to some variation, both individual 
and geographic. No attempt will be made 
here to discuss this variation which affects all 
parts of the mouse equally or nearly so. We 
will consider only those variations in color 
which result in one part of the mouse be- 
coming strikingly different in color from the 
other parts. 
A considerable number of house mice in 
California have the under parts separated in 
color from the upper parts. The upper parts 
retain the color of the common house mice of 
the region, while the under parts become col- 
ored either white, creamy buff, reddish buff, or 
intermediate tints between these colors and 
the color of the under parts of the unmodified 
house mouse. These colors of the under parts, 
where present, are sharply separated from the 
color of the back and sides at a definite line. 
In about half the specimens this line is em- 
phasized by the addition of a narrow stripe of 
pale fulvous. 
In all, seven house mice with white under 
parts, two with creamy buff under parts and a 
much larger number with reddish buff under 
parts have been taken up to the present time. 
Enough intermediate stages between the vari- 
ous colors have been found so that it becomes 
certain that these grade into one another and 
therefore are probably the product of the same 
What these 
factors are we can make no attempt to con- 
sider with the amount of data at hand. All at- 
tempts to correlate these color variations with 
eranial or bodily differences have been fruit- 
less. 
factor or factors of variation. 
SCIENCE 
835 
One peculiar specimen shows an oval white 
spot about one half inch long on the middle of 
the belly. In every other case noted the 
whole of the under parts are affected alike. 
The area covered by the modified color—except 
in this one instanee—is just about the same 
as the area covered by the white in the com- 
mon white-footed mouse (Peromyscus manicu- 
latus gambelz). 
The earliest record of any variation in the 
color of the ventral surface of the house mouse 
of California is furnished by a specimen in 
the Stanford University Museum collected 
April 3, 1898, by J. M. Stowell at Palo Alto. 
This specimen shows reddish-buff under parts 
sharply marked off from the color of the back. 
During November of 1907, Joseph Dixon took 
two specimens with white underparts and one 
with creamy buff under parts at Palo Alto. 
All other records of specimens with peculiar 
coloration are for the fall of 1910 and the 
spring of 1911. 
A large proportion of the mice showing the 
color variations on which this article is based 
have been taken at Palo Alto and Stanford 
University. Here a few were found on the 
salt marshes near San Francisco Bay and the 
rest in the houses and barns at Palo Alto and 
on the campus of the university. In some 
houses all the mice seem to be more or less 
modified in color, though not all in the same 
way or to the same degree. In other places 
most of the mice may have the typical house 
mouse coloration and only a few show any 
variation. From a lot of fifteen mice taken 
in two days at Stanford University, only one 
showed any considerable variation and this one 
was white on the belly. Nearly half the mice 
taken in the region show some modification of 
the color of the underparts. Besides the re- 
gion about Palo Alto, house mice showing 
variation in the color of the under parts have 
been taken during 1911 at Tipton, Tulare 
County, at Madera, Madera County, and at 
Pacific Grove, Monterey County. Of two 
house mice taken at Tipton, one shows a light 
creamy-buft ventral surface, while the other 
has the ordinary coloration of the house mouse. 
Of eleven house mice taken at Madera in a 
