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 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 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 



