And so with horses. Imported horses fail to breed healthfully in the damp provinces of India and Brazil, while horse sickness makes a clean sweep of them in many parts of Africa. It is probable, with horses as with men, that no sudden importation to regions outside their native zone of sunlight results in permanent healthy breeding. The imported strain dies out unless it is constantly renewed. Hordes of Asiatics with Dun horses have swept from time to time into Europe, and into India, but Dun horses are scarce in both regions, and do not exist in large numbers except in Scandinavia and in Katywar. {49} So the strong action of man in sudden floodings of Europe with Bays from the desert, Duns from the steppe, is outweighed by a stronger law of nature. With strains of horses as with tribes of man, the penalty for sudden migration from their native zone of light is gradual extinction. Yet is there one difference between Bays and Duns. The Dun is not worth renewing, and so dies out unnoticed. The Bay is worth breeding and so persists. PART IX. THE BRAND OF EUROPE. [Sidenote: The brand of Europe] In nature's immense and gentle processes, throughout the amazing story of the Europe horse, the bewildering actions of forgotten tribes of men, and the sun's own slow adjustments, a single force persists in branding the stock with a sign of ownership. A partial eclipse of the sun had made his figure that of the crescent moon. Standing under some oak trees, beside the road puddles made by recent rain, I noticed that the bars of reduced sunlight which came down through the leafage shone upon the little patches of water. The image of the crescent sun was reflected upside down. The bar of sunlight coming down through leafage acts as a lens to the sun's image. The {50} woodland glade is a camera. The coat of a woodland animal is coloured by the direct action of light, is sensitive to light, is a sensitized film for colour photography. To the peculiar reversed and condensed rays shining through leafage into the woodland camera, the coat of the horse responds, forming rings of deeper colour limited to the parts of the animal which are exposed to direct light. In the course of many generations, the rings become permanent and are known as dapples. The dappling in the dappled light of woodlands gives concealment both to hunting leopards and to hunted horses.