268 THE ORIGIN OF SPECIES 



developed from the tracheee; it is therefore highly proba- 

 ble that in this great class organs which once served for 

 respiration have been actually converted into organs for 

 flight. 



In considering transitions of organs, it is so important 

 to bear in mind the probability of conversion from one 

 function to another, that I will give another instance. 

 Pedunculated cirripeds have two minute folds of skin, 

 called by me the ovigerous frena, which serve, through 

 the means of a sticky secretion, to retain the eggs until 

 they are hatched within the sack. These cirripeds have 

 no branchiae, the whole surface of the bod}'' and of the 

 sack, together with the small frena, serving for respira- 

 tion. The Balanidse or sessile cirripeds, on the other 

 hand, have no ovigerous frena, the eggs lying loose at the 

 bottom of the sack, within the well-inclosed shell; but 

 they have, in the same relative position with the frena, 

 large, much-folded membranes, which freely communicate 

 with the circulatory lacunee of the sack and body, and 

 which have been considered by all naturalists to act as 

 branchiae. Now I think no one will dispute that the 

 ovigerous frena in the one family are strictly homologous 

 with the branchiae of the other family; indeed, they grad- 

 uate into each other. Therefore it need not be doubted 

 that the two little folds of skin, which originally served 

 as ovigerous frena, but which, likewise, very slightly 

 aided in the act of respiration, have been gradually con- 

 verted by natural selection into branchiae, simply through 

 an increase in their size and the obliteration of their 

 adhesive glands. If all pedunculated cirripeds had become 

 extinct, and they have suffered far more extinction than 

 have sessile cirripeds, who would ever have imagined that 



