CiiAr. VI. JIODES OF TRANSITION. 183 



may be converted into one for a widcly-tliffercnt purpose, 

 namely, respiration. Tlie swim-bladder has, also, been worked 

 in as an accessory to the auditory org'ans of certain hsh. All 

 physiolog^ists admit that the swim-bladder 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 swim-bladder 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 have descended by ordinary generation 

 from an ancient and unknown prototype, which was furnished 

 witii a floating apparatus or swim-bladder. We can thus, as I 

 infer from Owen's interesting description of these parts, under- 

 sta,nd the strange fact that every particle of food and drink 

 which we swallow has to pass over the orifice of the trachi^a, 

 with some risk of falling into the lungs, notwithstanding the 

 beautiful contrivance by which the glottis is closed. In the 

 higlier Vertebrata the branchiaj 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 tlunr 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, the branchiir; and dorsal scales of Anne- 

 lids are believed to be homologous with the wings and wing- 

 covcrs of insects, and it is not improbable that with our existing 

 insects organs, which at an ancient period served for respiration, 

 have actually been 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 another instance. Pedunculated cir- 

 ripcdes have two minute folds of skin, called by me the ovigerous 

 frena, Avhidi serve, through the means of a sticky secretion, to 

 retain the eggs until they are hatched within the; sack. These 

 cirri|)edes have no branclu;ii, the whole surface of the body and 

 of the sack, together with the small frena, serving for respira- 

 tion. Tiic 13alanid;\3 or sessih; 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 iiave, in the 

 same relative position with the frena, large, much-fokU'd mem- 

 branes, which freely communicate with the circulatory lacunixj 

 of the sack and body, and which have been considered to be 

 branchiaj by Prof. Owen and by all other naturalists who have 

 treated on the sul)ject. Now I think no one will dispute that 



