We have also emphasized the fact that a consider able proportion of the human race have not developed to the full capacity or to average mentality, but, on the contrary, have been arrested in various stages or at various levels. In the foregoing chapter we have shown the significance of this arrest in many practical affairs of life. These familiar faots of differences of intelligence among adults as well as between children of different ages have recently received new significance and great importance from the fact that we have learned to measure
and determine these mental levels with much accuracy. I t was the great French scientist, Alfred Binet (5 & 6),
that showed us how to measure intelligence, and it was his genius that gave us the first measuring scale. The practical application of the doctrine of mental levels de pends upon the ability to measure the intelligence of the individual. If intelligence cannot be measured, there is little practical value in knowing that there are levels of mental development; if it can be measured, then the sug gestions of the previous chapter are not only practical but of vital importance.