Page - 495 - in The Origin of Species

RUDIMENTARY ORGANS 495 lost the power of flying. Again, an organ, useful under cer- tain conditions, might become injurious under others, as with the wings of beetles living on small and exposed islands ; and in this case natural selection will have aided in reducing the organ, until it was rendered harmless and rudimentary. Any change in structure and function, which can be effected by small stages, is within the power of natural selection ; so that an organ rendered, through changed habits of life, use- less 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. Organs, originally formed by the aid of natural selection, when rendered useless may well be variable, for their variations can nolonger be checked by natural selection. All this agrees well with what we see under nature. Moreover, 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 tend to 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 the ad- joining parts, and their lesser relative size in the adult. If, for instance, the digit of an adult animal was used less and less during many generations, owing to some change of habits, or if an organ or gland was less'and less functionally exercised, we may infer that it would become reduced in size in the adult descendants of this animal, but would retain nearly its original standard of development in the embryo. There remains, however, this difficulty. After an organ has ceased being used, and has become in consequence much re- duced, how can it be still further reduced in size until the merest vestige is left; and how can it be finally quite obliter- ated? It is scarcely possible that disuse can go on producing any further effect after the organ has once been rendered functionless. Some additional explantion is here requisite which I cannot give. If, for instance, it could be proved that every part of the organisation tends to vary in a greater degree towards diminution than towards augmentation of size, then we should be able to understand how an organ