184 THE AMEBIC AN NATURALIST [Vol. LI 



at best rather gradual. Rubidus remains rubidiis and 

 sonoriensis remains sonoriensis, after several generations 

 of captivity in changed climates. But even the first cage- 

 born generation of each of my subspecies is found to be 

 highly modified by confinement, in respect to the mean 

 length of certain of the appendages. That this somatic 

 plasticity would be accompanied by a high degree of 

 germinal instability, as regards these parts, could not, of 

 course, be predicted in advance. But the frequent ap- 

 pearance of local differences of type renders it probable 

 that this is true. "Whether or not these local peculiarities 

 are due in some indirect way to environmental factors, 

 or whether they are due to "spontaneous" mutation, 

 need not concern us here. The main point to bear in mind 

 is the probability that the pelage color is somewhat more 

 stable in these mice than are the bodily proportions, 

 despite the fact that it is the former, rather than the 

 latter, which gives the clearest evidence of a definite cor- 

 relation with known factors of the environment. 



For the reason just stated, it is possible that the dif- 

 ferentiation of a new coloi* race might require fairly 

 rigid isolation; whereas local differences in some of the 

 measurable parts might arise in the presence of no other 

 barrier than the naturally slow rate of diffusion of a non- 

 migratory animal. As was remarked earlier, we have 

 reason to suppose that representative collections from 

 an indefinite number of localities would reveal the exist- 

 ence of statistically certain differences between the mice 

 of many of these localities. In most cases, it would prob- 

 ably be unjustifiable to assign these series to distinct races, 

 or other definite taxonomic groups, since it is likely that 

 perfect intergradation would be found between most of 

 them, and that the degree of difference would be largely 

 a function of the distance apart of their respective habi- 

 tats. 



These last remarks are, of course, largely conjectural. 

 Part of the author's present program consists in a care- 

 ful study of local differences of the sort here discussed. 



