Chap. VI.] MODES OF TRANSITION. 231 



one for a widely different purpose, namely, respiration. 

 The swimbladder has, also, been worked in as an acces- 

 sory to the auditory organs of certain fishes. All phys- 

 iologists admit that the swimbladder is homologous, 

 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 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 pro- 

 totype, 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, 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 gradu- 

 ally worked in by natural selection for some distinct pur- 

 pose: 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 impor- 

 tant to bear in mind the probability of conversion 

 from one function to another, that I will give another 

 17 



