Effects of External Conditions 143 
animals’ lives. This difference here amounts to 14.5° C., as com- 
pared with 16.6° during the winter of 1907-1908; and it might be 
at once inferred that in this fact we had a key to the difference of 
results. It must be noted, however, that, while the warm room 
temperature has been considerably lower for this period, during 
the later year (19.8° as compared with 23.7°), the cold room tem - 
perature has likewise been somewhat lower (5.3° as compared 
with 7.2°). Now a comparison of Tables 3 and 12 shows us that 
while the warm room tails agree pretty closely in length in the two 
series, the cold room tails are much shorter in the earlier one 
TABLE 13 
Series of 1908-1909: percentages of increase in the interval between successive measurements 
6 wrexKs* 3 MONTHS 
ABSOLUTE MEASUREMENTS | PERCENTAGES OF INCREASE 
| Weight pail,  \e Weta 2 |). erat 
MEN ee 12.604 67.19 Ohaktias 20.31 + 0.61 
Males...... | | | 
yCold’s.<: eres | iginicte), 60.11 | 58.86+ ? 22.79+ 0.46 
| SN EE EEE ol Soe SA ORs Neca Oem. a ee 
me f Wiatiirs cv en veal 12.663 68.95 on ACL OO RE | 15.81 + 0.42 
m ai 
Saini i Cold eC ENG | 11.889 59-49 40.75 ? | 18.53 0.66 
} | 
*See Table 12. 
Were these modifications simple functions of the temperature dif- 
ferences, we should not have expected such a state of things. Indeed 
the author has no explanation to offer for the striking difference 
between the two years’ results. 
A second set of measurements was made upon the same mice at 
the age of three months. The statistical treatment of these later 
figures is not yet quite complete. I have determined, however, 
the mean percentage increase in weight and tail length for each 
sex, both in the warm room and the cold room lots. Since every 
33 This is the figure for the room in which the thermograph was kept, and in which most of the mic 
lived during the greater part of this period. For certain reasons, however, all of the animals were 
kept for a period of varying length (8 to 42 days—the last in the case of only one brood) in another 
room, having a mean temperature about 4° higher than that of the room first referred to. This fact 
of course complicates the situation somewhat. 
