RUDIMENTARY ORGANS 406 



further than by showing that rudiments can be pro- 

 duced ; for I doubt whether species under nature ever 

 undergo abrupt changes. I believe that disuse has been 

 the main agency ; that it has led in successive genera- 

 tions to the gradual reduction of various 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 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 con- 

 tinue slowly to reduce the organ, until it was rendered 

 harmless and rudimentary. 



Any change in function, which can be effected by 

 insensibly small steps, is within the power of natural 

 selection ; so that an organ rendered, during changed 

 habits of life, useless or injurious for one purpose, 

 might be modified and used for another purpose. 

 Or an organ might be retained for one alone of its 

 former functions. An organ, when rendered useless, 

 may well be variable, for its variations cannot be 

 checked by natural selection. At whatever period of 

 life disuse or selection reduces an organ, and this will 

 generally be when the being has come to maturity and 

 to its full powers of action, the principle of inheritance 

 at corresponding ages will reproduce the organ in its 

 reduced state at the same age, and consequently will 

 seldom affect or reduce it in the embryo. Thus we can 

 understand the greater relative size of rudimentary 

 organs in the embryo, and their lesser relative size in 

 the adult. But if each step of the process of reduction 

 were to be inherited, not at the corresponding age, but 

 at an extremely early period of life (as we have good 

 reason to believe to be possible), the rudimentary part 

 would tend to be wholly lost, and we should have a case 

 of complete abortion. The principle, also, of economy, 

 explained in a former chapter, by which the materials? 

 forming any part or structure, if not useful to the 



