The population of the earth has been persistently increasing throughout recorded history, but only within the twentieth century has its size become an important obstacle to orderly civilization. One of the problems created by this growth, which has proved to be of some mathematical interest, is that of congestion. On land and in the air, in vehicles and on foot, people now get in each others' way to an extent far surpassing that of any previous age. There may have been overcrowding in ancient Rome or Elizabethan London, but it can hardly have constituted the hazard, the inconvenience, or the expense which it does today. We see congestion not only in transportation, but in virtually every aspect of modern life: communication, urban development, commercial organization, reticulation of utilities, mass production, and perhaps even agriculture.