It may thus be understood how profoundly complex are the relations and correlations of living creatures, and how a disturbance of some of the links in these living chains may very extensively affect the other links. The different creatures referred to above are probably the simplest illustrations that can be given of protectively colored animals. For here we have green animals adapted to a green environment—the green foliage of trees or grass. This adaptation has been brought about by Natural Selection. Suppose, for instance, that the habitat of the _non-protectively_ colored _ancestors_ of the frogs was for any reason unduly crowded by their enemies, the snakes. The frogs would be hard pressed and much persecuted. Suppose that among the young of these frogs there were some that varied in the direction of grass-green or leaf-green colors, while others did not so vary, or possibly varied in the direction of even conspicuous colors, such as black or white. It is evident that the latter groups would be easily detected by the snakes and destroyed, while the green group would frequently escape notice and would thrive and procreate their kind. This process being repeated generation after generation, there would come a time when the given habitat would contain none but frogs with a protective coloration of green. All of these protective colorations of snakes, butterflies, and caterpillars, as well as of frogs, are adaptations. They are illustrations of the survival of the fittest; the survival of those best adapted to their environment—in short, Natural Selection. [Illustration: PLATE II.—_Kallima paralekta._ A butterfly of Sumatra illustrating the work of Natural Selection: C, butterfly with expanded wings (dorsal surface) which are conspicuously colored through Sexual Selection; B, same butterfly with wings closed (ventral surface) and presenting a close resemblance to a dead leaf, A, through the agency of Natural Selection. B, illustrates the Protective Resemblance of an animal to an inanimate object.]