Effects of External Conditions 133 
tively little, though enough, probably, to deprive this difference 
of any significance. No such obvious modifications as has been 
alleged for cats 77 is here evident. 
Measurements at 7 Months. ‘The subsequent history of the 
female mice varied considerably with different individuals, accord- 
ing to the exigencies of the breeding experiments. Each female 
from either lot, as she became pregnant, was transferred to a room 
kept at a temperature somewhat intermediate between the hot 
and cold rooms. If, as was commonly the case (see p. 116), her 
brood did not survive, she was taken back to the room from which 
she came. Thus, in the interval between February 6 and April 
I, a considerable proportion of the females were transferred back 
and forth between the warm or cold room and the “intermediate” 
room, in some cases more than once. I[ have not thought it worth 
while to compute the average duration of each set of conditions for 
the lot. On April 1, all the mice were moved to the “intermediate” 
room and the sexes separated. On May 1, they were paired again, 
but with the same unfortunate results. ‘The entire lot of females 
was killed and measured between June 4 and July 6. All were 
about 7 months of age at the time of killing, save for a few mothers 
of broods, which were allowed to remain with the latter till they 
were old enough to take care of themselves. [hese somewhat 
older individuals (74 to 8 months) have, however, been included 
in the table herewith given. As they were, with little doubt, all 
fully grown, this proceedure seems fair. 
The mean figures obtained for each lot of females is given in 
Table 9g. It herewith appears that the weight in the warm room 
lot is 2.4 per cent greater than in the cold room lot, the body two- 
tenths per cent longer, the tail 14.9 percent longer,*the foot 4.1 per 
cent longer, while the average ear length is practically equal in the 
27 Tt is true that in the case cited by Lydekker the elongation of the vibrisse was attributed to the 
darkness, rather than to the cold. 
8 Tt seems probable that, so far as the tail at least is concerned, the cold-room mice have departed 
from the more usual or normal condition, while the warm-room individuals have been little if any 
modified. Fifty-nine adult female mice, of unknown history, which were received by me during the 
present winter, had a mean tail length of 92.8 mm.; i. e., their tails were considerably longer than 
even the warm-room females of Table 7. It must be noted, however, that they were larger mice, 
having a mean weight of 26.6 gms. 
