Effects of External Conditions I21 
on the other hand, the increase in weight and in tail length have 
been 28.8 per cent and 10.3 per cent respectively. ‘There has thus 
been manifested a tendency toward equalization in respect to both 
of these characters, but especially in respect to tail length. * This 
appendage has undergone a percentage increase which has been 
more than half again as rapid in the case of the cold room (i. e., 
the shorter tailed) as in the warm room (i. e., longer tailed) ani- 
mals. Further evidence for such a general tendency toward the 
neutralization of early differences will be offered later. As regards 
variability, the standard deviation for tail length, both in the warm 
and cold room lots, has undergone a slight absolute decrease, 
TABLE 4 
Series of 1907-1908: Females 24 months old 
| WEIGHT TAIL LENGTH 
| 
| | | 
Warm (33) =| Cold (21) | Warm (33) Cold (21) 
se | 
WMeancaee ec Mere as 15.821 + 0.261 15.176+ 0.330 76.18 + 0.38 | 60.19 + 0.76 
Standard deviation....) 2.221 0.184 2.2364 0.233 Tee he) ae Very ||" = ais) Se (o)slye| 
| | 
notwithstanding a considerable increase in the average for each 
lot. The standard deviation for weight, on the contrary, has under- 
gone an increase in each lot. The relative variability (1. e., ratio 
of standard deviation to mean) has increased in one case (warm 
room), decreased in the other (cold room). 
TABLE 5 
Series of 1907-1908: Mated males 24 months old 
WEIGHT TAIL LENGTH 
| Warm (7) | Cold (6) | Warm (7) | Cold (6) 
Wea ninea reir ite fe li6842' = Ost OSy | 720. nI7 0.824 | 78.00 + 1.17 | 66.00 + 0.62 
No discussion of the “mated males” (Table 5)” is worth while, 
owing to the small number of individuals comprised. ‘These 
3 A certain degree of selection was exercised in picking out these males, the larger individuals 
of a brood being chosen for breeding purposes. This is shown by the difference in mean weight 
between the ‘‘mated males” and the ‘‘unmated males.” 
