The influence of Environment may be investigated in two main aspects. First, one might discuss the modern and very interesting question as to the power of Environment to induce what is known to recent science as Variation. A change in the surroundings of any animal, it is now well-known, can so react upon it as to cause it to change. By the attempt, conscious or unconscious, to adjust itself to the new conditions, a true physiological change is gradually wrought within the organism. Hunter, for example, in a classical experiment, so changed the Environment of a sea-gull by keeping it in captivity that it could only secure a grain diet. The effect was to modify the stomach of the bird, normally adapted to a fish diet, until in time it came to resemble in structure the gizzard of an ordinary grain-feeder such as the pigeon. Holmgrén again reversed this experiment by feeding pigeons for a lengthened period on a meat-diet, with the result that the gizzard became transformed into the carnivorous stomach. Mr. Alfred Russel Wallace mentions the case of a Brazilian parrot which changes its color from green to red or yellow when fed on the fat of certain fishes. Not only changes of food, however, but changes of climate and of temperature, changes in surrounding organisms, in the case of marine animals even changes of pressure, of ocean currents, of light, and of many other circumstances, are known to exert a powerful modifying influence upon living organisms. These relations are still being worked out in many directions, but the influence of Environment as a prime factor in Variation is now a recognized doctrine of science.[81] Even the popular mind has been struck with the curious adaptation of nearly all animals to their _habitat_, for example in the matter of color. The sandy hue of the sole and flounder, the white of the polar bear with its suggestion of Arctic snows, the stripes of the Bengal tiger--as if the actual reeds of its native jungle had nature-printed themselves on its hide;--these, and a hundred others which will occur to every one, are marked instances of adaptation to Environment, induced by Natural Selection or otherwise, for the purpose, obviously in these cases at least, of protection.