The eye is a still more striking case, as too much use injures or even destroys it; while specialities of vision, as long or short sight, are undoubtedly _innate_, and usually persist throughout life. So the wonderfully varied bills of birds cannot be conceived as having been modified by use, and are, in fact, unchangeable when once formed. Yet, as they vary largely in every species, they are readily modified, so as to become adapted to new conditions by the "survival of the fittest." Equally impossible is it to connect any use or disuse, or environmental action, in the production, the gradual development, or complete adaptation to their conditions of life of the outer coverings of almost all living things—the hair of mammalia, the feathers of birds, the scales or horny skins or solid shields of reptiles, the solid shells of molluscs, wonderfully ribbed or spined, whorled, or turreted, and infinitely varied in surface colour and markings. Even more conclusive are the facts presented by the vast hosts of the insect world, from the massive armour of the ever-present beetle tribe, more varied in form, structure, ornament, and colour than any other comparable group of living things, to the widely different lepidoptera, equalling, or perhaps surpassing, the whole class of birds in their marvellous grace and beauty, yet all utterly beyond any possible direct action of the environment or of use and disuse in their development, and their close adaptation to that environment. Organic nature is indisputably one and indivisible. It has been developed throughout by means of the fundamental _forces_ of life, of growth and reproduction, and the equally fundamental _laws_ of variation, heredity, and enormous increase, resulting in a perpetual adaptation in form, structure, colour, and habits to the slowly changing environment. These forces and laws are _universal_ in their action; they are demonstrably adequate to the production of the whole of the phenomena we are now discussing. We see, then, that over by far the greater part of the whole world of life any modification of external structure, form, or colouring during the life of the individual is impossible; while in the remainder its action, if it exists at all, is of very limited range. No adequate _proof_ of the inheritance of the slight changes thus caused has ever yet been given, and it is therefore wholly unnecessary and illogical to assume its existence and to adduce it as having any part in the ever-active and universal process of evolution.