Chap. VI. TRANSITIONS OF ORGANS. 191 



view is now generally held, a part of the auditory- 

 apparatus has been worked in as a complement to the 

 swimbladder. All physiologists admit that the swim- 

 bladder is homologous, or " ideally similar," in position 

 and structure with the lungs of the higher vertebrate 

 animals : hence there seems to me to be no great diffi- 

 culty in believing that natural selection has actually 

 converted a swimbladder into a lung, or organ used ex- 

 clusively for respiration. 



I can, indeed, hardly doubt that all vertebrate ani- 

 mals having true lungs have descended by ordinary 

 generation from an ancient prototype, of which we know 

 nothing, furnished with a floating apparatus or swim- 

 bladder. We can thus, as I infer from Professor 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 dis- 

 appeared — the slits on the sides of the neck and the 

 loop-like course of the arteries still marking in the em- 

 bryo their former position. 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 Annelids 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, 



