Effects of External Conditions 145 
ferred to the same room when at the mean age of II weeks. 
When measured later, at the mean age of 3 months, the differ- 
ence in tail length had diminished to 6.7 per cent. The data for 
this series have not yet been fully compiled. 
SUMMARY OF STATISTICAL DATA 
The more significant facts which may be disentangled from this 
mass of data may be briefly summarized as follows: 
1 The tail length (whether absouute or relative) was found to 
be very much less in the case of those mice which were reared at 
the lower temperature. 
2 This difference was not accompanied by any appreciable 
difference in the number of vertebre. 
3. The foot length was likewise considerably less in all of the 
cold room lots, though the differences were not so large as in the 
case of the tail. : 
4 In two series, at least, the mean ear length was found to be 
smaller at the lower temperature, but these differences are perhaps 
to be regarded as “accidental.”’ 
5 Body length was not affected with any degree of certainty; 
while the influence of temperature upon weight, although evident 
in certain cases, was inconstant, and seemed to depend upon sex. 
6 Differences in the average quantity (weight) of hair have 
been demonstrated for the only series in which the point has been 
tested. The cold room lot were found to have an average amount 
of hair which was 11.4 per cent greater absolutely and 13.6 per 
cent greater relatively (1. e., allowing for the dimensions of the 
animal) than that of the warm room lot. The number of indi- 
viduals was, however, small (43) and the variability was high. 
7 In another experiment a considerable difference in the aver- 
age number of hairs per unit of skin area was found among the 
male mice, the number being greater in the cold room individuals. 
Here, likewise, the high variability and small number of individ- 
uals render any conclusions doubtful. 
8 No constant differences in variability were observable be- 
tween the warm room and the cold room mice. 
g To what extent the modifications cited above have resulted 
