MODES OF TRANSITION 195 



side out, and the exterior surface will then digest and the 

 stomach respire. In such cases natural selection might spe- 

 cialise, if any advantage were thus gained, the whole or part 

 of an organ, which had previously performed two functions, 

 for one function alone, and thus by insensible steps greatly 

 change its nature. Many plants are known which regularly 

 produce at the 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 char- 

 acter of the species. It is, however, probable that the two 

 sorts of flowers borne by the same plant were originally dif- 

 ferentiated 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 im- 

 portant 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 vas- 

 cular 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 in- 

 dividual. 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 swimbladdcr in fishes is a good one, 

 because it shows us clearly the highly important fact that an 

 organ originally constructed for one purpose, namely, flota- 

 tion, may be converted into one for a widely different pur- 

 pose, namely, respiration. The swimbladdcr has, also, been 

 worked in as an accessory to the auditory organs of certain 

 fishes. All physiologists admit that the swimbladdcr is homol- 



