3H4 Life and Letters of Froiich ftolton 



luid which were submitted to you two years ago. A very little reflection made it clear that 

 family likenew was nothing more than a particular case of the wide mihji'ct of correlation, ami 

 that the whole of the reasoning already bestowed upon the special case of family likeni^ss wn 

 equally applicable to correlation in its most general aspect." (p. 14.) 



This passage justifi&s the assertion that Galton came firet to correlation 

 from the study of heredity; that, when testing Bertillon's system of indexing' 

 anthro|X)metric measurements, he discovered correhvtion afresh and then 

 sjiw that lx>th associations were capable of identical treatment, and onl\ 

 special cases of a far wider conception. 



But there was another point Galton now recognised, in dealing with 

 heredity he had compared the same characters; he now saw that in comparing 

 different characters we must take each in its own unit of variation, .ind that 

 when this is done the relations become strictly reciprocal'. In this paper 

 Galton states, probably for the first time, that measured in units of variation. 

 there is always 'regression,' i.e. that in these units the proportion that one 

 average deviation of one character is of a given deviation of a second is 

 always a proper fraction, (p. 15.) 



The next section of this chapter is headed "Variety," and Galton laments 

 that, while an immense amount of trouble is taken over measurements, an- 

 tliropologists devote their inquiries solely to the means of groups, passing 

 over the variety of the individuals in those groups with "contented neglect. 

 The whole section is so thoroughly characteristic of Galton 's attitude to 

 anthropology and of his genial sagacity that I cannot refrain from reproducing 

 it almost entirely; there is much still to be learnt from its perusal even forty 

 years after it was written. 



"It seems to be a great loss of opportunity when, after observations have Ix-en lalK)riousl\ 

 collected, and been subsequently discussed in order to obtain mean values from them, that tin 

 small amount of e.xtra trouble is not taken, which would determine other values whereb}' tn 

 express the variety of all the individuals in thoRe groups. Much experience some years back, 

 and much new experience during the past year, prove<l to me the ease with which variety 

 may be adequately expressed, and the high im|)ortance of taking it into account. There arc 

 numerous problems of special interest to anthropologists that deal solely with variety. 



There can be little doubt that most persons fail to have adequate conceptions of the order 

 liness of variability, and think it is useless to pay scientific attention to variety, as being, in 

 their view, a subject wholly beyond the powers of definition. They forget that what is confessedly 

 undefined in the individual may l)e definite in the group, and that uncertainty as regards tlir 

 one is in no way incompatible with statistical assurance as reganls the other. Almost every IkxIs 

 is familiar now-a-days with the constancy of the average in difTerent samples of the same lari." 

 group, but they do not often re^alise the complettMie-HS with which a similar stjitistical constain \ 

 permeat<'s the whole of the group. The mean or the average is practically nothing more thai 

 the middleniuHt value in a marshalled series. A constancy analogous to that of the mean charm 

 teriseti the values that occupy any other fractional position that we please to name such as tli' 

 10th per cent, or the '20th per cent.; it is not peculiar to the SOth per cent., or middleinoM 

 Htill lesB do they realise the fact that all Variety has a strong family likeness, by approximatini 

 mora or leas olooely to the normal type, which is that which mathematicians prove must be tir' 

 oonaeqaence of Variety being due to the aggr^^te efToct of a very large number of small ami 

 independent inlluences. 



Greater interest is attached to individuals who occupy positions towards either of the end 

 of a marshalled stories tliati to thost; who stand alx)Ut its middle. An average man is morall; 



' In modem language Oalton recognised that with standard deviations as units, tlj' 

 ' regresnion ' is equal to the correlation coefficient. 



