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JOURNAL OF MAMMALOGY 
has ever adopted the familiar pale hue, which seems to be an adjust- 
ment to the prevailing tones of the desert at large. Either way we 
take it, the argument seems to lead us into difficulties. 
I am prepared, too, for the disparaging comments of such biologists 
as regard the experimental method as the only key to scientific truth. 
All this field observation, I may be told, is beside the mark. I should 
have subjected my animals to experimental tests in the laboratory. 
As a matter of fact, I have done this very thing, not with Peromyscus 
crinitus, to be sure, but with various subspecies of maniculatus. I have 
thus far found no evident tendency toward convergence on the part of 
these several races, even after a considerable number of generations in 
captivity under identical life conditions. Possibly photometer tests of 
prepared skins will reveal a slight change of color, but this is not yet 
obvious to the eye. The direct effect of humidity or other physical 
agents upon pigmentation, if such exists, must manifest itself very 
gradually in the case of these mice. 
In conclusion, let me say that I make no claim that the single case 
which I have studied intensively affords a disproof of the effect of a 
lava background upon the coat color of every other mammal. But I 
do urge that the wholly negative result derived from this seemingly 
critical case, gives ground for reasonable skepticism, and that it throws 
the burden of proof upon those who have, thus far, offered us merely 
generalized impressions or very limited data. 
Scripps Institution, La Jolla, California. 
