Chap. VI.] MODES OF TRANSITION. 231 



one for a widely different purpose, namely, respira- 

 tion. The swimbladder has, also, been worked in as an 

 accessory to the auditory organs of certain fishes. All 

 physiologists admit that the swimbladder is homologous, 

 or " ideally similar " in position and structure with the 

 lun^s of the higher vertebrate animals : hence there is 

 no reason to doubt that the swimbladder has actually 

 been converted into lungs, or an organ used exclusively 

 for respiration. 



According to this view it may be inferred that all 

 vertebrate animals with true lungs are descended by 

 ordinary generation from an ancient and unknown 

 prototype, which was furnished with a floating ap- 

 paratus or swimbladder. We can thus, as I infer from 

 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 — 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 branchiae might 

 have been gradually worked in by natural selection for 

 some distinct purpose : for instance, Landois 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, 



