46 Variation and Correlation in Brain-Weight 
(h) The absolute sex differences are sensibly the same for both age groupings. 
We may conclude then, that whatever changes occur in the weight of the brain 
with advancing age act in such a way as to leave the absolute sex difference 
unchanged. 
These absolute differences may be converted into relative differences by 
expressing them as percentages of the and $ means. When so expressed they 
take the following form : 
Sef Differences in Belation to Means. 
Eace 
"Total " series 
" Young " series 
Percentage of 
<? mean 
Percentage of 
? mean 
Percentage of 
(? mean 
Percentage of 
¥ mean 
Swedes 
Bavarians 
Bohemians 
Hessians 
10-6 % 
10-5 % 
9-9 % 
9'5 7„ 
11-8 7„ 
11-7 7o 
10-97 "U 
10-5 7^ 
10-3 7, 
9-8 7„ 
10-0 7, 
8-9 7„ 
11-5 7 
10- 8 7„ 
11- 1 7c 
9-8 7, 
The relative differences confirm the conclusions reached from the examination 
of absolute differences. The result that the changes which take place in the 
weight of the brain during adult life affect both sexes to an equal degree relatively 
seems to be of some importance. It implies that the factors which bring about 
the gradual lowering in the weight of the brain with advancing age are organic in 
a strict sense, and not dependent on environmental conditions. 
The same thing is shown if the relative differences expressed in another way, 
namely by taking the ratio of male to female mean, are examined. For our series 
these ratios are as follows : 
Sex Ratios. Means. Brain-weight. 
Eace 
" Total" series 
" Young " series 
Swedes 
1-118 
1-115 
Bavarians 
1-117 
1-108 
Bohemians 
1-110 
1-111 
Hessians 
1-105 
1-098 
The sex ratios for a number of other characters are given in the following table 
for comparison. The values in this table are taken from a table in Pearson's 
Chances of Death *. 
* Vol. I. p. 374. 
