DIFFICULTIES OF THE THEORY 151 



from the vegetable kingdom: plants climb by three distinct means, 

 by spirally twining, by clasping a support with their sensitive ten- 

 drils, and by the emission of aerial rootlets; these three means are 

 usually found in distinct groups, but some few species exhibit two 

 of the means, or even all three, combined in the same individual. 

 In all such cases one of the two organs might readily be modified 

 and perfected so as to perform all the work, being aided during 

 the 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 swim-bladder 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 different purpose, namely respira- 

 tion. 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 posi- 

 tion and structure with the lungs of the higher vertebrate animals: 

 hence there is no reason to doubt that the swim-bladder has ac- 

 tually 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 swim-bladder. 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 swal- 

 low 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 bran- 

 chiae 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 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 con- 

 verted 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 



