THE STREET SCENE in too many central sections of too many American cities suggests the existence of a wholly new variation on an ancient problem: homelessness. In prior years, before vagrancy laws were discovered to be unconstitutional, homeless people—usually male and of advanced years—were kept out of sight in a Skid Row section of their city. They are no longer out of sight, and their numbers are now unprecedented. Their aggressive behavior and bizarre appearance—often compounded by mental instability and chemical addictions frighten and discomfit urban residents, who react by proclaiming their hometowns Calcuttas.  These apprehensions are stimulated by organizations, ostensibly concerned with helping the homeless, that enthusiastically condemn America’s alleged heartlessness. But America’s cities are not yet Calcuttas in which whole families dwell permanently on the pavement.