MODES OF TRANSITION. 165 



tive suddenness in the character of the species. It is, how- 

 ever, 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 branchiae that breathe the air dissolved 

 in the water, at the same time that they breathe free air 

 in their swim-bladders, 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 sensi- 

 tive 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, com- 

 bined 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, 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 ani- 

 mals : 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 description 

 of these parts, understand the strange fact that every par- 

 ticle of food and drink which we swallow has to pass over 

 the orifice of the trachea ? with some risk of falling into the 



