No. 615] INHER1TAXCK IX I'KROMYSCV 



197 



(1) Weight and (2) body length are not dealt with di- 

 rectly in the present comparisons. The former is an in- 

 dex of metabolic condition as well as of size (/. e., length). 

 Captive mice, for example, are commonly fat in compari- 

 son with wild ones. Body length is of little significance in 

 comparing two groups of mice, unless we know, either 

 that the animals are all of the same age, or that the limit 

 of growth has been reached by all of them. These things 

 are frequently impossible to determine. 



(3) Tail length is dealt with, both as an absolute value 

 and as a percentage of the body length. If absolute tail 

 lengths are to be compared in two groups of animals, the 

 comparison can only be made between animals of approxi- 

 mately the same body length. My practise is to divide 

 each series into a number of size-groups, differing by 

 only two millimeters of body length. Group 80-81 of 

 one series is then compared with group 80-81 of the 

 other, group 82-83 with group 82-83, etc. The graphs 

 (Figs. 6 and 7 and 9-12) are based upon this procedure. 

 Each "curve" connects the means of the size-groups of 

 each series of animals, the abscissas representing body 

 length, the ordinates the character under comparison. 

 In order to eliminate very young mice, group.- having 

 body lengths of less than 80 mm. are omitted. Even so, 

 it is likely that most of the animals in the lower groups 

 of the series are immature, but this fact in no way affects 

 the validity of the comparisons. 



It will be seen, from an inspection of the figures (6 and 

 7), for both males and females, that, as regards tail length, 

 the Eureka mice (rubidus) stand in a class by themselves. 

 In comparison with the wide interval between this long- 

 tailed race and the other three races here considered, the 

 latter differ but slightly among themselves. It is evi- 

 dent, none the less, that the La Jolla animals have some- 

 what longer tails than do those from Berkeley or Victor- 

 ville, while the last two agree fairly closely in their mean 

 condition. 



Relative tail length, I e., the length of the tail expressed 



