DIFFICULTIES OF THE THEORY 257 



progress of modification by the other organ; and then 

 this other organ might be modified for some other and 

 quite distinct purpose, or be wholly obliterated. 



The illustration of the swimbladder in fishes is a good 

 one, because it shows us clearly the highly important fact 

 that an organ originally constructed for one purpose, 

 namely, flotation, may be converted into one for a widely 

 dift'erent 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 posi- 

 tion and structure with the lungs of the higher verte- 

 brate 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 dis- 

 appeared — but in the embryo the slits on tiie sides of the 

 neck and the looplike 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 distinct purpose: for in- 

 stance, Landois has shown that the wings of insects are 



