Read the excerpt from "Opening Ceremonies of the New York and Brooklyn Bridge" by Hon. Seth Low, Mayor of the City of Brooklyn. Fourteen years ago a city of 400,000 people on this side of the river heard of a projected suspension bridge with incredulity. The span was so long, the height so great, and the enterprise likely to be so costly, that few thought of it as something begun in earnest. The irresistible demands of commerce enforced these hard conditions. But Science said, "It is possible," and Courage said, "It shall be!" To-day a city of 600,000 people welcomes with enthusiasm the wonderful creation of genius. Graceful, and yet majestic, it clings to the land like a thing that has taken root. Beautiful as a vision of fairyland it salutes our sight. The impression it makes upon the visitor is one of astonishment, an astonishment that grows with every visit. No one who has been upon it can ever forget it. This great structure cannot be confined to the limits of local pride. The glory of it belongs to the race. Not one shall see it and not feel prouder to be a man. What is the central idea of this excerpt?