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155 
correlations, were it not that the prime appears to vary sensibly from one character 
to another. In other words, the "human prime" is purely relative to the special 
organ considered. There appears to be also no period of prime, but rather on the 
average an instant of prime, the time-graph of the character not being horizontal 
for any very sensible period. In the determination of the prime we ought to 
proceed by repeated measurements of the same series of individuals, and then 
reducing the individual time-graphs to a mean time-graph. This, of course, is 
absolutely impossible in the case of brain-weight and other internal organs. We 
are in these cases thrown back on the measurements of populations at different 
ages, the population at one age not consisting of the same individuals in whole or 
even in part as the population of another age. This method is admittedly de- 
fective, for it tacitly dismisses the whole question of natural selection*, and may 
lead us into strange pitfalls f; but it is often the only possible, and occasionally 
— since anthropometers will hardly work entirely for the future — the only practical 
method of approximating to the prime. 
(10) We are able to a certain extent now to answer a problem which has 
much exercised anatomists. Has the woman relatively to the man less brain- 
weight, or is the absolute difference in brain-weight merely the result of the 
different sizes of the two sexes J There is, we think, only one proper method 
of answering this (juestion. We must ask : What would be the brain-weight of 
a man having all his physical characters identical with that of the mean woman ? 
Would he have more or less average brain-weight ? Converselj^ given a woman 
with all her physical characters identical with those of the mean man, would she 
have more or less brain-weight ? Now unfortunately we do not know all the 
physical characters in either case, but we can judge from age, stature and the 
diametral product. 
Using the multiple regression eqiiation (i)" we find that : 
The Englishman of the same age, stature and diametral product as the mean 
woman has 1235 grs. brain-weight, or only 10 grs. more than the average woman. 
Further using (1)^ we find that: 
The Englishwoman of the same age, stature and diametral product as the mean 
man has 1315 grs. brain-weight, or only 13 grs. less than the average man. 
But we are not certain that this even is the limit of difference because the 
woman reaches her prime rather sooner than the man and declines more rapidly. 
It would be really more reasonable to compare the man physically like the 
woman, when at his prime, and the woman physically like the man when at her 
prime. Brain-weight primes are so little determined at present tliat it seems 
impossible to do this. But we think that there is probably as in other physical 
* The selective death-rate in man is probably GO to 80 p. c. See Biometrika, Vol. i. p. 74. 
t Pfitzner's conclusions drawn from the dead are probably to be largely discounted for this reason : 
see Biometrika, Vol. iii. p. 465. 
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