138 MODES OF TRANSITION. [CHAP. VI. 



is, however, probable that the two sorts of flowers borne by the 

 same plant were originally differentiated by finely graduated steps, 

 which may still be followed in some few cases. 



Again, two distinct organs, or the same organ under two very 

 different forms, may simultaneously perform in the same individual 

 the same function, and this is an extremely important means of 

 transition : to give one instance, there are fish with gills or bran- 

 chiae that breathe the air dissolved in the water, at the same time 

 that they breathe free air in their swimbladders, this latter organ 

 being divided by highly vascular partitions and having a ductus 

 pneumaticus for the supply of air. To give another instance from 

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

 spirally twining, by clasping a support with their sensitive tendrils, 

 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 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 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 exclu- 

 sively 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, under- 

 stand 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 con- 

 trivance by which the glottis is closed. In the higher Vertebrata 

 the branchias 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. 



