340 Trans. Acad. Sci. of St. Louis. 
the mean height throughout her growth. In that case, the 
law of growth for the type at a deviation of +1d from the 
mean is her law of growth. Otherwise, she is an exception 
and practical regulations deduced from the law for the type 
cannot be safely made binding on her. This knowledge, as 
has just been said, is furnished by the individualizing method, 
while the generalizing method is of no assistance in this matter. 
The application to individuals of the law of growth of the 
mean is a subject of immediate practical interest. The con- 
nection between theory and practical affairs is here unusually 
short and clear. Were this application possible, the devia- 
tions of children from the laws of normal growth could be 
quickly recognized and largely overcome, the evil effect of 
over study could be watched and intelligently combated, and 
systems of education, no longer exacting of all that which 
should be exacted only from the mean, could be rationally 
adapted to the special needs of the exceptionally weak and 
the exceptionally strong. 
These beneficent reforms, it is at present believed, must 
await the slow collection of data by the individualizing 
method. If it is indeed true that the laws of growth deter- 
mined for the mean cannot be used for the individual until 
the individualizing method has established the probability 
of each individual deviation remaining constant throughout 
the period of growth, then a generation must elapse — so 
slow is the gathering of data by this method— before the 
necessary knowledge is in our hands. I hope to show that 
this long waiting is unnecessary, and that the data collected 
by the generalizing method may be used, in a way hitherto 
unrecognized, for the making of standards by which the 
deviation of an individual from the mean of his age may be 
seen to be normal or abnormal. 
Let the problem be clearly understood. The question is: 
this boy or girl is above or below the mean height, or weight, 
etc., of his or her age. How shall it be known that this 
deviation is normal or abnormal? There has been hitherto 
no satisfactory reply to this question. A vague and partial 
answer is possible after two measurements separated by at 
least a year’s interval. If the deviation is the same, or very 
