THE AMERICAN NATURALIST [Vol. XLIX 



ency for these mice to be modified in the direction of 

 gambeli, this tendency is not sufficiently great to be de- 

 tected through any mere gross comparison, based upon 

 qualitative characters. Unfortunately, these two races 

 differ significantly in color characters only, so that an 

 exact quantitative test of this question is not here possible. 

 But I am now rearing P. m. rubidus at La Jolla, an ex- 

 periment which would seem to be much more promising 

 of decisive results than the one just described. 



Enough has been done, therefore, to prove that the color 

 differences between sonoriensis and gambeli are at least 

 in a large degree germinal and independent of environ- 

 mental influences acting during a single lifetime. 

 Whether or not they are wholly germinal, and if so, 

 whether they can resist change for an indefinite number 

 of generations, remains to be learned. Suppose that we 

 had a shifting of the mean to the extent of one per cent., 

 or even ten per cent., in the direction of gambeli. Amid 

 all the natural variability, we certainly should not be able 

 to detect such a change with the unaided eye. 



In interpreting the facts that I have offered as to the 

 relative fixity of these color differences of Peromyscus, 

 we should have due regard for various other experiments 

 which show that environmental influences may produce 

 notable changes of coloration in the lifetime of an indi- 

 vidual. As especially comparable with these tests of my 

 own, though differing completely in the outcome, may be 

 cited the experiments of Beebe. 19 This author produced 

 a marked increase of pigmentation in the feathers of three 

 species of birds by rearing them in an atmosphere of 

 abnormally high humidity. 



Beebe 's experiments seem to show that pigmentation 

 may in some cases be altered during a single lifetime by 

 changes of humidity. The generally known facts of geo- 

 graphic distribution show that there is in nature a dis- 

 tinct correlation between pigmentation and humidity. 

 My own experiments show that these geographic differ- 



™Zoologica, N. Y. Zool. Soc, Sept. 25, 1907. 



