172 ON THE ORIGIN OF SPECIES 



gwimbladder into a lung, or organ used exclusively foi 

 respiration. 



1 can, indeed, hardly doubt that all vertebrate 

 animals having true lungs have descended by ordinary 

 generation from an ancient prototype, of which we 

 know nothing, furnished with a floating apparatus or 

 swimbladder. We can thus, as I infer from Professor 

 Owen's interesting description of these parts, under- 

 stand the strange fact that every particle of food and 

 drink which we swallow has to pass over the orifice 

 of the trachea, with some risk of falling into the lungs, 

 notwithstanding the beautiful contrivance by which 

 the glottis is closed. In the higher Vertebrata the 

 branchiae have wholly disappeared — the slits on the 

 sides of the neck and the loop-like course of the 

 arteries still marking in the embryo their former posi- 

 tion. But it is conceivable that the now utterly lost 

 branchiae might have been gradually worked in by 

 natural selection for some quite distinct purpose : in 

 the same manner as, on the view entertained by some 

 naturalists that the branchiae and dorsal scales of Anne- 

 lids are homologous with the wings and wing-covers of 

 insects, it is probable that organs which at a very 

 ancient period served for respiration have been actually 

 converted into organs of 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 one more instance. 

 Pedunculated cirripedes 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 cirripedes 

 have no branchiae, the whole surface of the body and 

 sack, including the small frena, serving for respiration. 

 The Balanidae or sessile cirripedes, on the other hand, 

 have no ovigerous frena, the eggs lying loose at the 

 bottom of the sack, in the well-enclosed shell ; but 

 they have large folded 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 



