332 
Francis B. Sumner 
»Warm« figure 
The two equal 
Males . 
Females 
larger 
21 
22 
0 
1 
The same difference between the sexes in this respect is shown 
even more forcibly by the figures derived from the later measure- 
ments. 
Thus far we have treated these groups as of equal value in our 
computations. From our table it will be seen, however, that the 
groups differ greatly in respect to the number of individuals com- 
prized, and in respect to the magnitude of the differences shown. I 
have computed the probable errors of the averages for those seven 
size-groups which are large enough to make this worth while ^j. Tak- 
ing into account the three characters (tail, foot and ear) for the seven 
groups, we have, accordingly, twenty-one probable errors for each 
of the contrasted sets (>warm« and »cold«). By a little figuring it 
may be shown that in twelve of these 21 cases the difference between 
the two contrasted averages is two 2) or more times as great as the 
probable error of that difference; in one case the difference is over 
three times its probable error, and in three cases it is over four 
times its probable error. Furthermore, it is important to note that 
in none of the exceptional cases (i. e. those in which the cold-room 
descendants have longer peripheral parts) is the difference between 
the averages as high as two times its probable error ^). The signifi- 
cance of these facts will be appreciated by anyone familiar with 
statistical methods. 
Diagrams (Figures 6 and 7) have been constructed permitting 
of a comparison between the two contrasted sets of mice, with res- 
pect to the mean length of tail, foot and ear, for each of the size 
groups. The insignificance of the differences in the exceptional groups 
as compared with those conforming to the rule, is still further em- 
phasized by these diagrams. 
The question naturally arises : How do these differences between the 
warm-room and cold-room descendants compare in amount with the 
differences which were shown by the parents as a direct result of the 
1) I. e. those in which both sets (>warm« and >cold<) consist of ten or 
more individuals apiece (in one exceptional case, one set contains only nine). 
2) Here including one case in which it was very nearly twice as great. 
3) In one case it is practically equal, in another it is considerably less. 
