Porter — The Growth of St. Louis Children. 293 
shall be employed in the present work. In doing so it will be 
necessary to give a brief account of the matters concerning 
which agreement is general as well as those which are in dispute. 
It is acknowledged generally that the method of Quetelet 
furnishes a middle value typical of the series from which 
it was drawn, for example the middle weight of boys of 
the same age, nationality and social condition is the typical 
weight of boys of that age and class; and it is further 
acknowledged that the increase in the middle value from year 
to year expresses the law of growth of the type. It follows 
that the middle value of those who stand at any deviation from 
the middle value of the whole number is the type of that 
degree of deviation from the type of the whole number and 
that the increase in the middle value at the same degree of 
deviation at each age in the period of growth expresses the law 
of growth of the type at that degree of deviation. Thus the 
curves of percentile grades printed below express the growth 
of the typical St. Louis school-boy and girl. The type ata 
certain deviation from the mean of an age will show the same 
degree of deviation from the mean at any subsequent age; 
for example a type-boy in the 75 percentile grade at age 6 
will throughout his growth be heavier than 75 per cent. of 
boys of his own age. Percentile curves are of course not 
necessarily parallel. The type of the 50 per cent. who exceed 
the middle value of the whole number has a law of growth 
characteristic of tall boys and different from that of the type 
of the 50 per cent. who fall below the middle value of the 
whole number. 
The application of the middle value to individuals has not 
yet been mentioned. It is here that controversy finds its 
hold. The relation of the individual to the type is not known. 
It is not known whether a boy who at age 6 is heavier than 
75 per cent. of boys at his age will at age 18 be heavier than 
75 per cent. of boys at that age. Some anthropologists 
believe that there is at least probability that children remain 
in the same percentile grade throughout life, while others 
dissent from this view. In truth the development of the 
individual has been little studied, and it is therefore not yet 
possible to state his probable future growth. 
