Chap. XIV. mid Aborted Organs. 401 



rendering organs rudimentary. It would at first lead by slow steps 

 to the more and more complete reduction of a part, until at last it 

 became rudimentary, — as in the case of the eyes of animals in- 

 habiting 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 will have aided in re- 

 ducing 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, 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. Organs, originally formed by the aid of natural selec- 

 tion, when rendered useless may well be variable, for their vari- 

 ations can no longer 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 adjoining 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 reduced, 

 how can it be still further reduced in size until the merest vestige 

 is left ; and how can it be finally quite obliterated ? It is scarcely 

 possible that disuse can go on producing any further effect after 

 the organ has once been rendered functionless. Some additional 

 explanation is here requisite which I cannot give. If, for in- 

 stance, it could be proved that every part of the organisation tends 

 to vary in a greater degree towards diminution than towards aug- 

 mentation of size, then we should be able to understand how an organ 

 which has become useless would be rendered, independently of the 



2 D 



