Ti2 Francs B. Sumner 
above (p. 101). It seems worth while to present the individual 
figures obtained for these animals, since this is the only series 
with which the counting method was employed. In Table 2 each 
figure in the left-hand columns represents the number of hairs on 
a circular disk of skin 1.5 mm. in diameter. ‘Two figures are given 
for each individual, based upon two disks of skin taken at a dis- 
tance of I cm. to the right and the left respectively of the mid- 
dorsal line. The sum of these two is stated in heavy type. 
It will be noted that the mean number of hairs for the cold room 
males is 305.23 upon the two disks, that for the warm-room males 
being 267.08."* Here, then, is a difference of 14.3percent. Whe 
number of individuals is small, it is true, and the probable errors 
are large. Even granting the constancy, however, of such a 
difference, between two lots of mice thus treated, it is not neces- 
sary to conclude that there has been an actual increase in the 
number of (developed) hairs per unit of skin surface. If the 
warm-room individuals be supposed to have slightly thinner skins 
than those of the cold-room lot, the greater degree of stretching in 
the former (see p. 101) would result in a less dense distribution of 
hair upon its surface. But whether a difference so produced 
would be as high as 14 per cent may well be questioned. Again, 
it must not be supposed that I am urging this difference in the 
density of the coat of hair as an instance of permanent morpho- 
logical change. It may be due merely to a difference in the 
rate at which the hair is shed. ‘This point will be discussed later. 
The averages for the hairs of the female mice are not far from 
equal in the two contrasted lots. It has already been noted that 
the females exhibited a much smaller difference in tail length 
than did the males. 
Series of 1907-1908 
During the winter of 1907-1908 the experiments were conducted 
on a much larger scale than previously, the conditions employed 
were such as were calculated to result in the production of greater 
8 The number of hairs per square millimeter of skin may be readily computed, since the area of 
each disk was (approximately) 1.767 sq. mm. Thus the mean number for the cold room males is 
86.4, that for all the mice comprised in the table is 85.0, etc. 
