120 Frances B. Sumner 
figures shows that both the males and the females are divided 
quite sharply into these two groups. The number of animals 
is so small, however, that this phenomenon may be accidental. 
It will be referred to later. 
3. In the case of both sexes, the variability both in weight and 
in tail length is considerably greater for the cold room lot dian for 
the warm room lot. 
4. The mean weight of the males (irrespective of history) is 
considerably greater (9.6 per cent) than that of the females; the 
tail length seems to bear no constant relation to sea! does the 
variability of either character. 
Measurements at 24 Months. At the age of 24 months the mice 
were mated. At about the same time (when 75 to 78 days old) 
the measurements previously made (weight and tail length) were 
repeated. ‘The number of females surviving to this age was 54 
(33 warm + 21 cold). The number of males reserved for breed- 
ing was 13 (7 warm + 6 cold). There remained 43 males (22 
warm + 21 cold) which were not needed for breeding purposes. 
They were therefore killed at this time, and subjected to a con- 
siderably greater number of measurements than had hitherto been 
employed. At this period, therefore, there are to be considered 
three groups of individuals (each group consisting, of course, of a 
warm room and a cold room half) : (1) the females; (2) the mated 
males; (3) the unmated males. 
Referring to Table 4, it will be seen that among the female mice 
the warm room lot are 4.2 per cent heavier than the cold room 
lot, while the tail length of the former is 26.6 per cent greater. 
Comparing the mean figures here given with those of the table for 
42, days, it will be found that in the cold room lot the mean weight 
has undergone an increase of 33.1 per cent during the interval 
between the two measurements, while the mean tail length has 
undergone an increase of 16 per cent.” In the warm room lot, 
* Of course this increase in the mean has not exactly the same value as the average individual 
increase. The latter figure cannot be given for the present series, since the mice had not been marked 
so as to be individually distinguishable, although the members of each litter were identified by a 
brood number. Inasmuch as there had been but two deaths in the interval between the measure- 
ments, we are dealing with practically the same group in each case. 
