Effects of External Conditions 10g 
discussed at all fully. The mean temperatures in the two rooms, 
during the period of the experiment, were 24.9° C. and g.1° C. 
(76.8° F. and 48.4° F.) respectively. No further analysis of the 
temperature conditions seems worth while in the present experi- 
ment. The humidity was not at any time determined. ‘[wenty- 
one mice (13 males and 8 females) were reared in the cold room; 
20 mice (12 males and 8 females) in the warm room. ‘The ani- 
mals were not subjected to the differing temperatures until they 
were about three weeks old (21 + days). Previous to that time, 
the undivided lots had been reared under similar conditions. Each 
lot comprised individuals from 7 different broods, each of the latter 
having been divided into two portions destined for the warm and 
cold rooms respectively. About half the stock, consisting of the 
broods earliest obtained, were subjected to the experimental 
conditions for a period of 106 days, the remainder for a period of 
83 days. At the expiration of these terms the mice were paired 
for breeding purposes, and the two contrasted lots were trans- 
ferred toa single room having a temperature somewhat interme- 
diate between those previously employed. None of the animals 
were killed immediately after this transfer, while the females were 
kept until they had reared their broods. ‘The interval between the 
discontinuance of the temperature differences and the killing of 
the animals varied from 15 to 55 days. ‘Thus the material was 
far from homogeneous. Nevertheless the figures obtained seem 
worth recording. (Table 1.)” 
The difference in tail length, in the males, at least, was often 
obvious without measurement, and it must be regarded as a signif- 
cant one, even without such overwhelming corroborative evidence 
as is offered later. In the case of the females, the difference is 
much smaller, though it is greatly in excess of the probable errors. 
No further analysis of this table seems called for. 
In the present series, the number of hairs per unit of skin area 
was computed for each mouse, according to the method described 
7 Here and elsewhere the number of individuals measured for any given character is indicated by 
the figure in parentheses at the head of each column. In some cases individual tail measurements 
have been thrown out, where the tip of the organ had obviously been lost through accident. I trust, 
however, that it is not necessary to urge that no merely ‘‘exceptional” figures have been rejected 
