14S MODES OF TRANSITION. [Chap. VI. 



"because it shows us clearly the highly important fact that an organ 

 originally constructed for one purpose, namely, flotation, may he 

 converted into one for a widely different purpose, namely, respiration. 

 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 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 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, 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 branchia? 

 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 trachea? ; 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 Balanida) 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 lacuna? of the sack and body, and which have been 

 considered by all naturalists to act as branchias. Now I think no 

 ono will dispute that the ovigerous frena in the one family are 

 strictly homologous with- the branch.' a) of the other family ; indeed, 



