I am indebted to you for giving me this opportunity to put before you certain tentative conclusions at which I have arrived as a result of working at the problem of physiological instability on a group of 525 children between the ages of 6 and 14. Like so many investigations of this type, it began on a presumption that was not borne out by further experience, and because of this it caused me to revise certain views I held on the subject.