230 MODES OF TRANSITION. [Chap. VI. 



same time differently constructed flowers ; and if such 

 plants were to produce one kind alone, a great change 

 would be effected with comparative suddenness in the 

 character of the species. It 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 branchiae 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 



