Porter — The Growth of St. Louis Children. 345 
weight and other physical dimensions of an individual are 
normal in relation to height, and it has been pointed out that 
this normal relation constitutes health. It follows that the 
normal amount of labor cannot be exacted without injury 
from those in whom this equilibrium is wanting. These 
facts must therefore be taken into account in a rational 
school system, and it should now be made plain how this is to 
be done. 
All systems of education have for their object the largest 
possible development of individual minds. In large schools 
the tasks by which this development is promoted are those 
which secure from the child of mean ability its maximum 
mental output. In practice they are determined by examina- 
tions. Hence the existence in every educational institution 
of classes based on the mental output of the mean pupil, and 
related to age only in that the output fixed as the standard of 
any class is necessarily found more often at a certain age than 
at other ages. Thus there exists a mean age for each class; 
the greater number of pupils at any age is found in the same 
class, while some have advanced beyond, and others, equally 
old, have not yet come so far as this class. On an average, 
those who have advanced beyond the greater number of their 
age are precocious, that is, possess more than the mean eapac- 
ity for mental labor, while those who are less advanced are 
dull, possessing less than the mean capacity. It has been 
demonstrated that there is a physical basis for precocity and 
dullness:' when numbers sufficiently large for statistical 
purposes are employed, it is seen that precocious pupils are 
of greater mean weight, height, etc., than the mean pupils 
and that the latter are heavier and taller that the dull. The 
mental output is therefore directly related to the physical 
condition of the pupils. The mean height, weight, girth of 
chest, etc., in any grade is the mean physical development 
corresponding to the mental output of the grade. It follows 
that those who do not possess this development cannot with- 
out abnormal strain do the work exacted in this grade. On 
the other hand, pupils who possess more than the mean 
1 See Chapter VIII, page 335 et seq. 
