230 MODES OP TRANSITION. [Chap. VL 



time differently constructed flowers; and if such plants 

 were to produce one kind alone, a great change would 

 he effected with comparative suddenness in the charac- 

 ter of the species. It is, however, probable that the 

 two sorts of flowers borne by the same plant were origi- 

 nally 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 swimhladders, this 

 latter organ being divided by highly vascular partitions 

 and having a ductus pneumatieus for the supply of air. 

 To give another instance from the vegetable kingdom: 

 plants climb by three distinct means, by spirally twin- 

 ing, 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 obliter- 

 ated. 



The illustration of the swimbladder in fishes is a 

 good one, because it shows us clearly the highly im- 

 portant fact that an organ originally constructed for 

 one purpose, namely, flotation, may be converted into 



