196 ORIGIN OF SPECIES 



ogous, or "ideally similar" in position and structure with the 

 lungs of the higher vertebrate animals : hence there is no 

 reason to doubt that the swimbladder has actually been con- 

 verted into lungs, or an organ used exclusively for respi- 

 ration. 



According to this view it may be inferred that all verte- 

 brate animals with true lungs are descended by ordinary gen- 

 eration from an ancient and unknown prototype, which was 

 furnished with a floating apparatus or swimbladder. We can 

 thus, as I infer from Owen's interesting description of these 

 parts, understand 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, notwith- 

 standing the beautiful contrivance by which the glottis is 

 closed. In the higher Vertebrata the branchise have wholly 

 disappeared — but in the embryo the slits on the sides of the 

 neck and the loop-like course of the arteries still mark their 

 former position. But it is conceivable that the now utterly 

 lost branchise might have been gradually worked in by nat- 

 ural selection for some distinct purpose : for instance, Lan- 

 dois has shown that the wings of insects are developed from 

 the tracheae; it is therefore highly probable 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 

 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 of the sack, together with 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, within the well-enclosed shell ; 

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

 large, much-folded membranes, which freely communicate 

 with the circulatory lacunae 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 



