454 RUDIMENTARY ORGANS. Chap. XIII. 



of growth, but in order to excrete horny matter, as that 

 the rudimentary nails on the fin of the manatee were 

 formed for this purpose. 



On my view of descent with modification, the origin 

 of rudimentary organs is simple. We have plenty of 

 cases of rudimentary organs in our domestic produc- 

 tions, — as the stump of a tail in tailless breeds, — the 

 vestige of an ear in earless breeds, — the reappearance 

 of minute dangling horns in hornless breeds of cattle, 

 more especially, according to Youatt, in young animals, 

 — and the state of the whole flower in the cauliflower. 

 We often see rudiments of various parts in monsters. 

 But I doubt whether any of these cases throw light on 

 the origin of rudimentary organs in a state of nature, 

 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 

 easily be modified and used for another purpose. Or 

 an organ might be retained for one alone of its 



