Origin of Species Variorum

organs, until they have become rudimentary,— as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced by beasts of prey to take flight, and have ultimately lost the power of flying. Again, an organ useful under certain conditions, might become injurious under others, as with the wings of beetles living on small and exposed islands; and in this case natural selection would continue slowly to reduce the organ, until it was rendered harmless and rudimentary. ↑

Any change in structure and function, which can be effected by insensibly small steps, is within the power of natural selection; so that an organ rendered, through changed habits of life, useless or injurious for one purpose, might .. be modified and used for another purpose. An organ might, also, be retained for one alone of its former functions. An organ, originally formed by the aid of natural selection, when rendered useless, may well be variable, for its variations can no longer be checked by natural selection. ↑ At whatever period of life either disuse or selection reduces an organ, and this will generally be when the being has come to maturity and has to exert its full powers of action, the principle of inheritance at corresponding ages will reproduce the organ in its reduced state at the same mature age, but will seldom affect .. it in the embryo. Thus we can understand the greater size of rudimentary organs in the embryo relatively to its other parts, and their lesser relative size in the adult. But if each step of the process of reduction were to be inherited, not at a corresponding age, but at a very early period of life, the rudimentary part would tend to be ... wholly lost, and we should have a case of complete abortion. The principle, also, of the economy of organisation, explained in a former chapter, by which the materials forming any part, ..