What a great difference there is in brilliancy of colouring between birds and the furry creatures. How the plumage of a cardinal, or indigo bunting, or hummingbird glows in the sunlight, and reflects to our eyes the most intense vermilion or indigo or an iridescence of the whole gamut of colour. On the other hand, how sombrely clad are the deer, the rabbits, and the mice; gray and brown and white being the usual hue of their fur. This difference is by no means accidental, but has for its cause a deep significance,--all-important to the life of the bird or mammal. Scientists have long known of it, and if we unlock it from its hard sheathing of technical terms, we shall find it as simple and as easy to understand as it is interesting. When we once hold the key, it will seem as if scales had fallen from our eyes, and when we take our walks abroad through the fields and woods, when we visit a zoological park, or even see the animals in a circus, we shall feel as though a new world were opened to us. No post offices, or even addresses, exist for birds and mammals; when the children of the desert or the jungle are lost, no detective or policeman hastens to find them, no telephone or telegraph aids in the search. Yet, without any of these accessories, the wild creatures have marvellous systems of communication. The five senses (and perhaps a mysterious sixth, at which we can only guess) are the telephones and the police, the automatic sentinels and alarms of our wild kindred. Most inferior are our own abilities in using eyes, nose, and ears, when compared with the same functions in birds and animals. Eyes and noses are important keys to the bright colours of birds and comparative sombreness of hairy-coated creatures. Take a dog and an oriole as good examples of the two extremes. When a dog has lost his master, he first looks about; then he strains his eyes with the intense look of a near-sighted person, and after a few moments of this he usually yelps with disappointment, drops his nose to the ground, and with unfailing accuracy follows the track of his master. When the freshness of the trail tells him that he is near its end he again resorts to his eyes, and is soon near enough to recognise the face he seeks. A fox when running before a hound may double back, and make a close reconnaissance near his trail, sometimes passing in full view without the hound's seeing him or stopping in following out the full curve of the trail, so completely does the wonderful power of smell absorb the entire attention of the dog.