XX.] ' COMPARATIVE MORPHOLOGY, 251 



wings, because the rudimentary bones in question are Organs if 

 useless : in other words, they are not at work, and therefore n^ust^be 

 are incapable of improving. Such organs as these, which aborted, 

 have become rudimentary through disuse, are said to be 

 cibortccl. But instances may be pointed out, of organs that 

 are in the act of acquiring a new function ; these may be 

 called nascent organs for their new purpose. The best 

 instance of this that I know of, is that of the lepidosiren, Nascent 

 an animal concerning which it is a debated point whether lepido- 

 it ought to be classed as a fish or a Batrachian, and which ^n'en. 

 has its lungs in a state that presents an evident transition 

 from the swim-bladder of a fish to the lungs of an air- 

 breathing Vertebrate. 



-■o 



In this chapter I have endeavoured to show, that homo- 

 logical resemblances between organs which are unlike in 

 form and function are explicable only on the supposition 

 that they are proofs of a common descent. In the next, I 

 intend to commence the attempt to explain, by means of 

 the laws of habit and correlation, what the course of 

 modification in certain cases has been. 



