When we once realise that whether in attacking or avoiding an enemy it is in most cases a great advantage to all animals to be hidden from view, and that each creature has arrived at this advantage by slow inheritance, so that their colours often exactly answer the purpose, how wonderful becomes the gray tint of the slug, the imitation of bark in the wings of the buff-tip moth, the green and brown hues of the eatable caterpillars, the white coat of the polar bear, and the changing colour of the arctic fox, the ermine, and the ptarmigan, as winter comes on! And when, on the other hand, we find badly-tasting creatures such as ladybirds and some butterflies, or stinging animals like bees and wasps, having bright colours, because it is an actual advantage to them to be known and avoided, we see that in studying colour alone we might spend a lifetime learning how the winners in life’s race are those best fitted for the circumstances under which they live, so that in ever-changing variety the most beautifully-adapted forms flourish and multiply. Then if we turn to the skeleton and the less conspicuous framework of the body, the flippers of the whale, the manatee, or the seal, doing the work of a fish’s fin and yet having the bones of a hand and arm, reveal a whole history to us when we have once learned the secret that in the attempt to increase and multiply no device is left untried by any group of animals, and so every possible advantage is turned to account. Next, the wonderful instincts taught by long experience give us a whole field of study. We see how frogs and reptiles, and even higher animals such as marmots, squirrels, shrews and bears, escape the cold and scarcity of food in winter by burying themselves in mud, or in holes of trees or caves of the earth till spring returns; and while we find alligators burying themselves in cold weather in America, we find crocodiles, on the contrary, taking their sleep in the hot dry weather in Egypt because then is their time of scarcity. Then we learn that the birds avoid this difficulty of change of climate in quite another manner. They with their power of flight have learned to migrate, sometimes for short distances, sometimes for more than a thousand miles, so that they bring up their young ones in the cool north in summer, when caterpillars and soft young insects are at hand for their prey, and lead them in the winter to the sunny south where food and shelter in green trees are always to be found. So long indeed has this instinct of migration been at work, that often we are quite baffled in trying to understand why they take this or that particular route for their flight, because probably, when the first stragglers chose it, even the areas of land and water were not divided as now, so that we must study the whole history of the changing geography of the earth to understand the yearly route of the swallow or the stork.