128 THE RATE OF GROWTH [ch. 



The amount of variability is bound to differ from one race or 

 nationality to another, and we find big differences between the 

 Americans and the Japanese, both in magnitude and phase (Fig. 22). 



If we take not merely the variability of stature or -weight at a 

 given age, but the variability of the yearly increments, we find 

 that this latter variabihty tends to increase steadily, and more and 

 more rapidly, within the ages for which we have information; and 

 this phenomenon is, in the main, easy of explanation. For a great 

 part of the difference between one individual and another in regard 

 to rate of growth is a mere difference of phase — a difference in the 

 epochs of acceleration and retardation, and finally a difference as to 

 the epoch when growth comes to an end ; it follows that variabihty 

 will be more and more marked as we approach and reach the period 

 when some individuals still continue, and others have already ceased, 

 to grow. In the following epitomised table, I have taken Boas's 

 determinations * of the standard deviation (ct), converted them into 

 the corresponding coefficients of variabihty {olM x 100), and then 

 smoothed the resulting numbers: 



Coefficients of variability in annual increments of stature 



The greater variabihty in the girls is very marked f, and is 

 explained (in part at least) by the jnore rapid rate at which the girls 

 run through the several phases of their growth (Fig. 24). To say that 

 children of a given age vary in the rate at which they are growing 

 would seem to be a more fundamental statement than that they 

 vary in the size to which they have grown. 



Just as there is a marked difference in phase between the growth- 

 curves of the two sexes, that is to say a difference in the epochs 

 when growth is rapid or the reverse, so also, within each sex, will 

 there be room for similar, but individual, phase-differences. Thus 

 we may have children of accelerated development, who at a given 



* Op. cit. p. lo48. 



I That women are on the whole more variable than men was argued by Karl 

 Pearson in one of his earlier essays: The Chances of Death and other Studies, 1897. 



