Effects of External Conditions IOI 
CHARACTERS CHOSEN FOR MEA SUREMENT 
In the selection of these characters it was of course desirable to 
choose one or more which might be supposed to be influenced 
adaptively under the conditions of the experiment. = Quantity 
of hair was therefore chosen as oneof the values to be deter- 
mined. ‘Theoretically the hair might be subjected to quantitative 
treatment in three different ways: (1) by determining the average 
length of the hairs on each pelt, based upon a considerable num- 
ber of individual hairs, so taken as to be representative of the 
entire lot; (2) by finding the number of hairs on a given unit of skin 
area; (3) by ascertaining the wezght of the entire pelage or hair-coat 
ofeach mouse. The first method was very soon found to be imprac- 
ticable, owing to the fact that the hairs are seldom straight enough 
for the purpose, being commonly curved or zigzag. The second 
and later the third methods were, however, employed more or less 
advantageously. For the determination of the number of hairs 
in a given unit of skin, the latter, immediately after removal from 
the freshly killed animal, was subjected to a stretching process 
which was as far as possible identical for each pelt. Pinch clamps 
were fastened at six points on the margin of the skin, and to these 
were attached fine cords which passed outward over pulleys 
arranged around the skin in a circle. Each cord bore a 100-gram 
weight. ‘The pelt was thus gently stretched for five minutes, after 
which a large cork was pressed lightly against its central region, 
and the skin pinned to it by a circle of 16 pins. The outlying 
portions of the pelt were now cut away, and the circular area which 
was left pinned to the cork was allowed to dry. After a variable 
interval, depending on convenience, a small disk of the skin was 
cut out by means of a tool devised for the purpose. ‘This consisted 
of a steel tube, 1 mm. in diameter, sharpened at one end into an 
edge. In use, it was pressed rather lightly against the inner sur- 
face of the skin and rotated until the latter was completely cut 
through. ‘The resulting disk of skin was fastened to a black tile 
—hair uppermost, of course—by means of glue. The hairs were 
now shaved off with a sharp knife and counted under a low power 
lens. Two disks from symmetrically placed points on the pelt 
