Why Then and Not Earlier

Not a list of inventions with dates beside them. Every sheet asks the question a list cannot: why did this arrive when it did, and not three hundred years before? The printing press waited on paper. The light bulb waited on somewhere to plug it in. The wheeled suitcase waited five thousand years for smooth floors and a long walk to the gate. And the reason two people so often invent the same thing in the same year is that neither of them was the cause.