988 
Journal of Argicultural Research 
Vol. XXVIII, No. 10 
being-22.5 and 20.5, or a difference of 9.2 per cent. These figures are real and 
not fortuitous, for there are seven males with a foot length of 23 mm., but no 
female with feet so long. The individual variation in foot length is considerable, 
and this measurement may be of only slight diagnostic value when but a few 
specimens are available; but in series it is important. 
LENGTH OF EAR 
It is a number of years now since the writer became convinced of the worthless¬ 
ness of the measurement of length of ear when taken from the fresh specimen in 
the field. A little experimenting demonstrated that without some sort of ap¬ 
paratus giving an even pull, it is practically impossible for a collector to arrive at 
any degree of accuracy or uniformity in taking length of “ear from crown,” and 
“ear from notch” is almost as bad in this respect. In fact, the writer obtained 
some startling results, due to the facility with which the ear tip may be stretched 
from the head, and he thereupon abandoned the practice of taking this measure¬ 
ment. 
In the present study the writer endeavored to use some sort of ear measure¬ 
ment taken from the dried skin, but, because of the usual distortion of this member 
while drying, has concluded that no reliance can be placed upon any figures 
resulting, and that it is better to omit this. A careful examination of the ears 
of the skins at hand indicates that there is no appreciable variation either individ¬ 
ually or due to age. 
WEIGHT 
In any consideration of the weights of mammals one must remember that he 
is dealing with a measurement of mass, in three dimensions really, whereas all 
other measurements taken are of one dimension only. It is thus inevitable 
that there will be variation of a sharper degree than is usual in measurements of 
length; but this in itself is no argument against the importance and value of 
recording weights. 
It is not usual for the Microtinae to lay up a great store of fat; in fact it is 
only rarely, if at all, that any definite quantity of fat is to be found upon them. 
For this reason the weights of individuals of like size do not fluctuate so decidedly 
as is the case with mammals which habitually take on a great supply of fat in antici¬ 
pation of a period of hibernation. The weight of a Zapus taken in fall would be 
out of all proportion to that of an individual of like age taken in spring. The 
weight of this sort of mammal is thus of real value only when accompanied by data 
concerning its condition. The meadow mice of the present series were very 
uniform in this respect, and almost all contained a moderate quantity of food 
without having the stomachs greatly distended. 
The individual variation in the weights of males is not excessive, considering 
that this is a measurement of mass. Variation with age is much more pro¬ 
nounced, of course. The average weight of 19 males is 52.7 gm., with maximum 
of 70.7 for old ones down to 35.6 for small young adults. This is a variation 
among sexually mature examples of 35 gm., or 67 per cent of the average weight. 
This amount of variation between individuals of such ages is undoubtedly exces¬ 
sive for rodents in general, but it is a well-known fact that it is usual in this sub¬ 
family. In fact, it may sometimes be greater, for veritable giants among meadow 
mice are not infrequently encountered, although there is none in the series under 
discussion. It will be noted that the largest male is considerably heavier than 
any of the females, even than those containing large embryos. 
The weights of females are much less reliable and are more variable individually 
than those of the opposite sex because of the presence of embryos in a large num¬ 
ber. The average weight of the 32 females is 48.2, with extremes of 62.2 and 
34.8, a difference of 27.4 gm. This size variation among examples that are sex¬ 
ually mature of 56.4 per cent of the average is considerably smaller (11 per cent) 
