Ability witJt the Size and Skajjc of the Head. 337 



of the head changes, and we are compelled to reduce our measurements 

 to what they would be if the children were all of one age. Taking the 

 length of head, Dr. Alice Lee formed a correlation table between 

 length of head and age for 1856 boys, ranging from 4 to 19 years 

 of age. The mean head-length was taken for each year of age, and 

 thus a curve of average size of head of boys from 4 to 19 obtained. 

 This curve presented several marked features, notably an apparent 

 period of rest in growth during the 12th year.* These features will 

 be more fully discussed on another occasion, but the rest in the 12th 

 year is referred to here as a reason why we have chosen the 12th year 

 as a standard age to which we may refer all our measurements. The 

 growth of the average boy from every year up to the 12th year, and 

 from the 12th up to every later year, was now determined. These 

 growths were then added to or subtracted from the head-length 

 measured on the individual in order to deduce from the observed size 

 of the boy's head his approximate size of head in the 12th year. This, 

 of course, is far from legitimate ; it supposes that each individual boy 

 grows like the average boy, which we know is not the fact. Still it 

 will not lead us very far astray for the purpose of the broad classifica- 

 tion we are proposing.! 



A precise!}' similar process was adopted for the breadth of head. 

 The 12th year was still chosen for the standard age, although in this 

 case the growth-curve from 4 to 19 years exhibits no period of rest, 

 being almost exactly a straight line. % 



In our school observations the following classification of intelligence 

 was adopted (it having been shown by experiment to give fairly con- 

 cordant results when the same children were classed by independent 

 observers) : Quick-Intelligent, Intelligent, Slow-Intelligent, Slow, Slow- 

 Dull, and Very Dull. For the purposes of our present investigation,, 

 we grouped the first two categories together and the remaining four 

 together. We will call the first group Intelligent and the second Slotc, 

 and these may be taken to roughly correspond to our first division of 

 Honours and Poll men when dealing with Cambridge graduates, 

 results on this point obtained by Dr. \V. Pfitzner, ' Zeitschrift fur Morphologic u. 

 Anthropologie,' vol. 1, 1899, p. 372. 



* A like but less marked rest in the 12th year is indicated in W. T. Porter's 

 curves for growth of head in boys : see "The Growth of St. Louis Children," 

 ' Trans. Acad. of Science of St. Louis,' vol. 6, pp. 2G4-380, Plate 32. 



t An examination of Mr. Porter's curves of growth for the 25th, 50th, and 75th 

 percentile grades (loc. cit., Plate 32) seem to indicate (by the parallelism of these 

 curves) how small really is the correlation between amount of growth and actual 

 length of head. 



J This is again in keeping with Mr. Porter's results for ages 6 to 17 ; see loc. 

 cit., Plate 33. His percentile curves are again approximately parallel, and thus- 

 favour the view that amounts of growth are largely independent of size. Why 

 there is rest in growth of length but not in growth of breadth is an interesting 

 problem. 



