Effects of External Conditions E27 
the females and will be discussed later. It must be added, how- 
ever, that in the present case the difference in weight between the 
two groups has increased rather than diminished. In respect 
to variability, two of the four standard deviations comprised in the 
table show an absolute decrease, one of the others indicates a 
slightly lessened variability, while in the fourth case there is an 
increase, both absolute and relative. Not much importance is 
to be attached to these latter comparisons, however. 
Returning to a consideration of the figures presented in Table 6 
it is seen that the weight is 5.3 per cent less in the warm room lot 
than in the cold room lot; while the body is seven-tenths of one 
per cent longer and the tail 23.9 per cent longer. Passing to the 
new measurements (not applicable to living animals), the aver- 
age length of the (left) ear is 1.7 per cent greater in the warm room 
lot; that of the foot 3.3 per cent greater. The average weight of 
hair (see p. 102) for the cold room mice is 11.4 per cent greater 
than that of the warm room individuals. It has seemed worth 
while to represent graphically the frequency distributions of these 
characters (Figs. 4 to 9). The difference in weight between the 
two sets of mice (Fig. 4) cannot with any certainty be regarded as 
a significant one. ‘The difference in body length (Fig. 5) is too 
trivial to be taken into consideration. 
In striking contrast, however, is the case for tail length (Fig. 6). 
Here there is no overlapping whatever of the polygons, while 
the modes are removed by a distance of 13 mm. ‘Two questions 
present themselves respecting this difference of tail length: (r) 
Does it involve an actual difference in the volume of the organ? 
And (2) does it involve a difference in the number of vertebrz ? 
In order to test the first question the diameter of the tail at its 
widest point was obtained by means of calipers for all the mice 
of this group. While this is a dificult measurement to make with 
any great accuracy, the figures are probably exact enough for 
present purposes. The averages for warm room and cold room 
animais are 2.94 mm. and 3.02 mm., respectively. There is 
thus the possibility of a slightly greater attenuation of the tail, 
accompanying its more rapid increase in length, but the former 
is certainly nothing like proportionate to the latter. There is 
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