82 
JOURNAL OF MAMMALOGY 
areas of pale sand, but appear to wander freely over the barest and 
blackest masses of lava rock, in which positions they were frequently 
trapped. 
Preliminary trapping revealed the fact that there were several species 
of mice on this field. By far the most frequent was Peromyscus crinitus 
stephensi, but P. maniculatus sonoriensis and two species of Perognathus 
were met with. 
Circumstances pointed to the first of the foregoing species as being 
best adapted for the test which I had in view. As is well known, this 
mouse shares the prevailing buff or sandy hue so characteristic of 
desert rodents. Furthermore, it has one very great advantage for 
present purposes, namely, that it is restricted in its habitat to rocky 
regions, in the crevices of which it finds its shelter. Here, then, we 
seemed to have the conditions favorable for a crucial natural experi- 
ment. This rather extensive lava field was surrounded on all sides by 
areas of sand and fine gravel. Trapping, undertaken to test this 
point, revealed the fact that the crinitus mice rarely, if ever, left the 
lava beds and strayed for any distance into the open desert. Live- 
traps, which I set for 175 “trap nights’ upon the desert areas adjacent 
to the lava field yielded but two specimens of crinitus, both of these 
within 150 feet of the lava. Furthermore, Mr. Hunt, during two 
weeks’ use of spring traps in this region, did not catch a single speci- 
men of this species away from the lava beds. In contrast to these 
negative results, it must be pointed out (1) that large numbers of mice 
of other species were taken in these sandy areas, and (2) that crinitus 
was extremely common throughout the lava field, where it predominated 
over all other small rodents. It would seem likely that for centuries, 
and perhaps for a vastly greater period, this colony of Peromyscus 
crinitus stephensi had been isolated by fairly rigid barriers from other 
mice of the same species.^^^ Since the statements of Merriam, Osgood 
and Goldman, cited above, relate to species and to localities concerning 
which no such claim of isolation is advanced, it would seem that in 
This useful unit of trapping activity is due to Grinnell (An Account of the 
Mammals and Birds of the Lower Colorado Valley, University of California 
Publications in Zoology Vol. 12, 1914, p. 92). 
11^ Doctor Crinnell suggests that “periodic eruptions” of these mice, due to 
over-population, might carry them, from time to time, across these barriers. I 
can only reply (1) that we have no knowledge of such migrations here; (2) that 
such a diffusion process is at least as likely to occur in those localities where a 
darkening effect of lava has been alleged. 
