June 7 , 1924 
Variation in Microtus Montanus Yosemite 
1015 
Variation of the mandible is considerable, but the characters are usually 
correlated with other portions of the skull, and it is considered that most man¬ 
dibular characters are relatively unimportant in systematic work when the skull 
proper is available. 
The patterns of the first lower and third upper molars are excessively variable, 
but this is individual, and practically no part of it is due to age. It is felt that 
an undue phylogenetic importance has usually been ascribed to these differences 
within a group of species, but that in such aggregations of individuals as the 
present they point to nothing more than unstable evolutional trends. 
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