Plain Facts for Old and Young by John Harvey Kellogg

8. _Failure of mental capacity_ without apparent cause should occasion

suspicion of evil practices. When a child who has previously learned readily, mastered his lessons easily, and possessed a retentive memory, shows a manifest decline in these directions, fails to get his lessons, becomes stupid, forgetful, and inattentive, he has probably become the victim of a terrible vice, and is on the road to speedy mental as well as physical ruin. Watch him.