In one of his best known books, Mr. Darwin brings out a fact which may be illustrated in some such way as this: Suppose a bird fancier collects a flock of tame pigeons distinguished by all the infinite ornamentations of their race. They are of all kinds, of every shade of colour, and adorned with every variety of marking. He takes them to an uninhabited island and allows-them to fly off wild into the woods. They found a colony there, and after the lapse of many years the owner returns to the spot. He will find that a remarkable change has taken place in the interval. The birds, or their descendants rather, have all become changed into the same colour. The black, the white and the dun, the striped, the spotted, and the ringed, are all metamorphosed into one—a dark slaty blue.