June 7,1924 
Variation in Microtus Montanas Yosemite 
979 
down to the smallest detail (save for the lachrymals), except for four that were 
very slightly damaged by the traps. 
Of this series but three specimens are definitely juvenile, the remainder being 
adults and subadults, all sexually mature. Such maturity can at once be 
distinguished in the field by an experienced collector, but not with certainty in 
the case of prepared specimens. Criteria are the general appearance of the animal 
before skinning, the condition of the pelage, and the development of the external 
genitalia. That such intangible points may be trustworthy in this regard is 
indicated by the fact that a majority of the very smallest females that are here 
designated as subadult contained embryos. 
These 66 skins with skulls consist of 25 males and 41 females. Unfortunately 
no record was kept concerning the sexes of the discarded individuals, and it is 
now impossible to state whether the relative proportion of the two sexes in the 
series at hand is representative of conditions in the colony at large. The writer 
would be exceedingly loath to adopt the hypothesis that female Microtus sub¬ 
stantially outnumber the males at birth, without evidence of a very conclusive 
character. Reference to field notes shows, however, that any disparity between 
the sexes in the series of different species of microtines that the writer has 
prepared has almost always been in favor of the females. This state of affairs 
can neither be ignored nor settled offhand. 
METHODS EMPLOYED 
In addition to extremely careful ocular comparison of the specimens, elaborate 
measurements of skulls have been taken by means of dial calipers of great 
accuracy. A list of skull measurements was selected which it was thought, 
after some trial, might prove to be of greatest value in the study of the sub¬ 
family Microtinae. These were made, tabulated according to sex, and arranged 
consecutively in descending sequence with reference to the condylo-basilar 
length as a basis. Taking the skull of the female with greatest condylo-basilar 
length as representing 100 per cent, and each of the other measurements of 
this skull as also representing 100 per cent, a list of percentages was made, 
tabulated/and plotted. It was thought that the actual plotting need be done only 
in the case of the more numerous sex. In the graph for body measurements 
and weights (fig. 2), also arranged according to the condylo-basilar dimension 
of the skulls, the averages for both sexes of the three body measurements and 
the weights have been taken as 100 per cent, each for its respective part. 
This ^treatment of measurements—plotted by percentage and proportion—is 
believed to give a much clearer perception of the relation of each dimension to 
every other one considered than does the usual treatment of the subject. The 
one objection to this method is that in reducing, or rather magnifying, to per¬ 
centage such small measurements as interorbital width, the coefficient of error 
is also necessarily magnified; but, nevertheless, such treatment allows one more 
easily to discern whether there be any correlation between the variations of the 
various portions of the skull. 
It is perhaps too trite to be worth stating that no series of animals can ever be 
exactly uniform, for there exist too many developmental factors to permit 
precise correlation between parts, save in very rare instances; hence, all that is 
possible is an attempt to locate correlative trends between various portions of the 
animal. 
In the accompanying graphs a mean or average line plotted between the 
perpendicular extremes would represent variation with age, while the extent to 
which the lines for each sex deviate from such an imaginary mean would signify 
individual variation. 
