336 
Francis B. Sumner 
There were at the later date 114 of the cold- room descendants, and 
only 84 of the warm-room descendants. The survivors all appeared 
to be in good health, however, and few or no deaths had occurred 
during the few weeks immediately preceding these measurements. 
At the time of the lirst series of measurements, each mouse had 
been measured upon arriving at a certain age (42 days). The later series, 
on the contrary, were made during a period of one week Septem- 
ber 5 — 11), and without reference to the age of the individual mice^). 
An approximately equal proportion of warm-room and cold-room des- 
cendants were, however, measured each day of this period. It was deter- 
mined later that the mean age of the two sets of individuals at the 
time of these second measurements was in each case almost exactly 
the same, i. e. about 105 days. The youngest mouse was 90 days old, 
the oldest being 113 days old. These extremes were exceptional, 
however, for 76 per cent of the cold-room descendants were between 
the ages of 103 and 110 days, while 87 per cent of the warm-room 
descendants were between the ages of 103 and 109 days. 
In order to exclude the possible influence of suggestion or un- 
conscious bias in determining these rather delicate caliper measure- 
ments, I adopted at this time the plan of keeping myself in igno- 
rance as to the parentage of each mouse until the latter had been 
measured. The animals were put into separate small cages, each 
bearing an identification mark upon the bottom. These cages were 
then » shuffled « by one of my colleagues in the laboratory. 
The mice were killed at the time of these latter measurements. 
For this reason it was possible to determine another character — 
body length — with accuracy. This is not feasible with living ani- 
mals, even when etherized. 
The gross averages for the two contrasted sets of individuals 
are presented herewith: 
Weight Body length Tail Foot Ear 
Cold-room descendants 18.56 86.703 81.162 17.8960 13.3386 
(114) 
Warm-room descendants 19.45 87.683 82.732 18.1427 13.5143 
(84) 
ij The latter — identified by marks, to be sure — had been mingled 
together in a few large cages, and it would have required much labor to pick 
out, each day, those which had arrived at the age desired. 
