From the foregoing brief outline it would seem that only a little exercise of reason is required of any one to see that natural selection operating in unlike environments would, in the course of many generations, produce from the same species types of animals (or plants) very unlike--not only unlike each other but unlike their parent species. At first these types derived from a common ancestor would be only varieties, but varieties are incipient species. Given sufficient time, and the intervention of natural barriers to prevent the crossing of extremes, and the creation of new species would be the natural result. As a vivid illustration of how the law of natural selection works, and of how great is the sum of the results of its operation accumulated through many generations, let us observe one of many modifications an animal undergoes in its struggle for existence. To the question, “Why are animals of the arctic regions white?” we ought to be able, with what we already know of natural selection, to give the answer. Imagine those lands of snow and ice originally inhabited by animals of all colors from white to black, or even of all shades of one color, brown. The animals of what colors or shades, among the flesh-eaters, would have the least difficulty in stealthily approaching their prey, and so, be most apt to survive in the struggle against starvation? And which among the animals preyed on would run the least risk of detection and so be most likely to escape destruction? The answer to both questions, of course, would be, “Those whose colors most nearly conformed to the snowy background.” Imagine this process of culling out the darker colors continuing for many generations and we can understand that the whiteness of all arctic animals would be the inevitable result. We can also see why most wild animals of our regions are of colors that best harmonize with the brown earth and dead leaves. Let us keep in mind the fact that while we were observing the modification of one character, all the favorable variations, however slight, in every other character that could be of the least advantage to those creatures were being added up as fast as they appeared. After several generations, if for any cause the environment were undergoing a radical change, or if a species had migrated from a widely different one, a greatly modified animal would be the result. However, there can be no modification in an animal perfectly adapted to its environment--provided the environment does not change. Every organ, and every other part of the body, internal and external, of every creature in the natural state is subject to modification by the law of natural selection, just as was the color of the hair or the feathers of polar animals.