Effects of External Conditions 103 
to insure the removal of fats, and then dried in the air. Each 
lot (the total pelage derived from a single pelt) was now put into 
a weighing bottle and transferred to a vacuum desiccator, until 
the weight was found to have become constant (usually for about 
four days). The final weight was recorded to the ten-thousandth 
of a gram, though only thousandths have been regarded in the 
present paper. Since a number of specimens were dealt with at 
once, the process did not require as much time as might be 
inferred from the number of steps involved. ‘The results obtained 
are presented below (pp. 129-131). 
Other characters chosen for measurement were: werght, length 
of body (from snout to anus), /ength of tail (from anus to tip), 
length of left ear (from the lower extremity of the incisura inter- 
tragica" to the tip of the pinna), and /ength of left hind foot (from 
heel to tip of longest toe). Certain additional determinations, 
which were made in special cases, will be referred to at the proper 
time. A uniform method of procedure was adopted in making 
each of these measurements. In order that my figures may be of 
service for comparison with those of other workers in this field, 
it seems desirable to detail these methods a little more fully. The 
weight was taken by means of a torsion balance sensitive to a few 
hundredths of a gram. ‘The tail length was obtained in two ways, 
according to whether the animal was living or dead when meas- 
ured. When measured living, the mouse was suspended by the 
tip of the tail, the forefeet being allowed to rest upon the table. 
A certain degree of stretching generally occurred, the amount 
of which was found to average somewhat over 2 mm. in the case 
of full-grown mice. Weight and tail length were the only char- 
acters which it was possible to determine accurately with living 
animals.” In measuring the tail of the dead mouse, and likewise 
the body length, the freshly killed animal was laid with the ventral 
surface uppermost upon a wooden board. A pin was passed 
through the roof of the mouth, thus securing the snout to the board. 
The latter was then held upright and the body allowed to dangle 
11 Or at least a point which would seem to be homologous with the part so named in the human ear. 
12 T have since found it practicable to measure the ear and foot, in the living mouse, by the use of 
ether (April, 1909.) 
