176 MODES OF TRAJSSITION, 



one for a widely different purpose, namely, respiration. 

 The swim-bladder has, also, been worked in as an accessory 

 to the auditory organs of certain fishes. 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 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 verte- 

 brate animals with true lungs are descended by ordinary 

 generation from an ancient and unknown prototype, which 

 was furnished with a floating apparatus or swim-bladder. 

 We can thus, as I infer from Owen's interesting descrip- 

 tion 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 

 intc the lungs, notwithstanding the beautiful contrivance 

 by which tho glottis is closed. In the higher Vertebrata 

 the branchi^ 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 dis- 

 tinct purpose: for instance, Laudois has shown that tho 

 wings of insects are developed from the trachea; it is there- 

 fore 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 func- 

 tion to another, that I will give another instance. Pedun- 

 culated 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 Balanid^ or 

 sessile cirripedes, on the other hand, have no ovigerous 

 frena, the eggs lying loose at the bottom of the sack, 

 within the well-inclosed shell; but they have, in the same 

 relative position with the frena, large, much-folded mem- 

 branes, which freely communicate with the circulatory 



