342 
Francis B. Sumner 
at a glance, the curves (Figure 11) which are based upon the aver- 
ages for the weight-groupts (sexes combined) show that for foot and 
ear length, at least, the same relations obtain as were previously 
portrayed in Figs. 7 and 9. 
It might indeed have been anticipated that the results derived 
from these later measurements would be less striking than those der- 
ived from the earlier ones^). To begin with, the number of indi- 
viduals was considerably reduced, they varied somewhat in age, and 
the mean size of the two contrasted lots differed. Then too, we have 
to reckon with the principle of the ^levelling down of initial differ- 
ences*, concerning which I have had considerable to say in an ear- 
lier paper. And lastly, it is possible that unconscious bias in the use 
of the calipers may have somewhat exaggerated the differences shown 
in the earlier series of measurements, although caution was, of course, 
taken to avoid this. 
Summary and Conclusions. 
1) Mice which were reared in a warm-room (about 21" C.) were 
found to differ considerably from those reared in a cold-room (about 
5° C.) as regards the mean length of the tail, foot and ear. These 
organs were found to be longer in the former than in the latter set 
of individuals. (The two sets probably also differed from one another 
in respect to the amount of hair, though this does not concern u& 
here.) 
2) These same differences were found to be manifested by the 
offspring of the warm-room and the cold-room parents, although the 
animals belonging to this second generation were all reared together 
in a common room, and exposed to identical temperature conditions. 
In the experiment here considered, there were 141 of the warm-room 
descendants, 145 of the cold-room descendants. 
3) These differences between the warm-room and cold-room des- 
cendants are revealed not only through a comparison of the gross 
averages for these three characters in the two contrasted sets of in- 
1) As a matter of fact, I had not yet analyzed the data from the earlier 
measurements at the time when the later ones were made, and at that time had 
no expectation that these interesting relations would be shown to exist. Indeed 
nothing was observed during the course of the measurements to justify such 
an expectation. 
