144 Francis B. Sumner 
mouse in the present year’s experiments is identified by a mark of 
its own, it has been possible to compute the rate of increase for 
each animal individually. In Table 13 are presented the mean 
percentages of increase for each character during the period in 
question. 
It will be seen from this table that in all four cases. the figure 
expressing the increase in a character is /arger in that group in 
which the previous absolute measurement had been smaller. It 
must be added, however, that in neither case is the figure for 
weight of any significance in this connection, despite the differ- 
ences between the averages. ‘The variability in the weight-increase 
has been enormous (ranging fromg per cent to 143 percent), so that 
the probable errors (not yet computed) are undoubtedly very large. 
For the growth of the tail, however, the case seems fairly certain. It 
w.ll be recalled that in the preceding series it was the figures for 
tail length which bore the strongest testimony to the principle 
of the leveling down of original differences. Reference to the 
temperature curves shows that here, as previously, the differences 
in the conditions have increased rather than diminished during 
this period. As regards the increase or decrease in variability 
within each lot, nothing can be said here, since the standard devia- 
tions have not been computed for the 3-months measurements. 
Supplementary. A yet later series of animals (born March 
1909), consisting of a larger number of individuals than any of 
the preceding lots, has yielded, after similiar teatment, the fol- 
lowing results: (1) a difference in tail length somewhat greater 
than that shown in the preceding series (16.7 per cent in the pres- 
ent case); (2) an indubitable difference (both absolute and rela- 
tive) in foot length, which applies equally to both sexes and 
fully confirms the earlier conclusions on this point; (3) a simi- 
lar difference in ear length, which, however, is of far less certain 
significance; (4) a difference in weight, both sexes being heavier 
in the warm room lots (cf. pp. 118,138 above),'although this differ- 
ence was greater for the females (11.3 per cent) than for the 
males (9.1 per cent). The temperature differences to which the 
animals were subjected had already begun to diminish at the 
time of the first measurement (42 days), and both lots were trans- 
