Effects of External Conditions 141 
ature conditions will be further discussed after the first table of 
measurements has been presented. ‘The mean relative humidity 
in the warm room (12 determinations) was 38.5 per cent, a figure 
very close to that of the preceding year. The humidity of the 
cold room (14 determinations) was 76.5 per cent, being thus consid- 
erably greater than that of the preceding year. ‘This is accounted 
for by the fact that the cold room used during the present year has 
been situated in a building which is entirely unheated. 
With respect to weight there is a remarkably close agreement 
between each of the four averages here presented and the cor- 
responding one of the preceding year. In each year’s series, the 
males are larger in the cold room lot, while the females are larger 
inthe warmroom lot. From the magnitude of the probable errors, 
however, we cannot feel sure of the validity of these differences.” 
Regarding tail length, it will be noted that the differences, 
while considerable, are very much smaller than those to be found 
in Table 3. The warm room males have tails which are 11.8 
per cent longer than those of the cold room lot. ‘This difference, in 
the case of the 1907-1908 lot, was 27.1. The warm room females 
show a mean tail length which is 15.9 per cent greater than that 
of the cold room individuals, as compared with a difference of 
33 per cent in the earlier lot. Thus, as in the preceding year’s 
measurements, we find that it is the females which have been 
modified most in this respect. But the amount of modification 
tor each sex has been less than half that which was earlier observed. 
Regarding variability no deductions of interest are to be drawn. 
No such evidence of a higher variability in the cold room lot as 
was previously noted is here to be found. In fact, the reverse is 
possibly true. 
The salient fact brought out by this comparison is the relatively 
small degree of modification in the tal length of the present lot 
of animals. ‘This fact is not satisfactorily explained by a com- 
parison of the temperature conditions for the two years. We find, 
it is true, in the later series a somewhat smaller difference in tem- 
perature between the two rooms during the first six weeks of the 
32 See supplementary note below. 
