RUDIMENTARY ORGANS 271 



It appears probable that disuse has been the main 

 agent in rendering organs rudimentary. It would at first 

 lead by slow steps to the more and more complete reduc- 

 tion of a part, until at last it became 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 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, 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 func- 

 tions. 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 dis- 

 use 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 inheri- 

 tance 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 under- 

 stand the greater size of rudimentary organs in the em- 

 bryo relatively to the adjoining parts, and their lesser 



