These experiments show that animals of almost proverbial density may learn with surprising quickness. English sparrows and other avian inhabitants of the city learn to live tranquilly along the busiest thoroughfares, exposed to all sorts of dangers, and subjected to what would be to many birds the most terrifying circumstances. Whizzing trolleys, tramping multitudes, and screaming engines have no terrors for them. They simply exercise the caution necessary to keep from being run over. They boldly build their nests right under passing elevated cars, where the roar is sufficient to scare the life out of an ordinary country bird. I have seen these testy little chaps sit and feed and jabber to each other in a perfectly unconcerned way within ten or fifteen feet of a thundering express train. They do not do these things from instinct: they _learn_ to do them. They know that a diabolical-looking locomotive is harmless, because they have seen it before; and they know that an insignificant urchin with a savage heart and a sling is not harmless, and they know it simply because they have previously had dealings with him. English sparrows will disappear completely from a neighborhood if a few of them are killed. Cats, dogs, horses—all animals, in fact—acquire during life a fund of information as to how to act in order to avoid harm and extinction. If they did not, they would not live long. And they do it just as man does it, by memory and discrimination, by retaining impressions made upon them, and acting differently when an impression is made a second, third, or thirteenth time.