Effects of External Conditions 131 
coat. I have therefore made an endeavor to compute the 
relative amount of hair, making allowance for the area of 
the skin—that is, | have obtained the ratio between the hair 
weight of each mouse and the square of its body length. The 
mean of these figures is 0.04501 mg. per sq. mm. for the cold 
room lot, 0.03959 for the warm room lot. According to this com- 
putation, the cold room mice have a relative amount of hair 13.6 
per cent greater (heavier) than the warm room ones. It 
must be admitted, however, that such a method of computa- 
tion is open to criticism. To estimate the relative skin 
areas of these mice by comparing the squares of their body 
lengths presupposes that they are, in the language of geom- 
Fig. 9 Series of 1907-1908: weight of hair (absolute) of ‘“‘unmated males” at 24 months (expressed in 
hundredths of a gram). 
etry, “similar solids,” which they are not. As a matter 
of fact, while the warm room mice have a slightly greater body- 
length than the contrasted group, they are lighter, on the aver- 
age, by nearly one gram, i. e., a difference of over 5 percent. They 
are probably somewhat less plump, therefore. I must add, 
however, that I do not believe any such slight difference 
of shape to be accountable for the difference in the amount 
of hair which is shown by the two groups of mice. The most 
serious criticism of these figures relates to the number of indi- 
viduals, which is confessedly too small to permit of our drawing 
any final conclusions in the presence of such high variability. 
