CiiAi-. VI.] MODES OF TRANSITION. 147 



an organ common to all the members of a class, for in this latter 

 case the organ must have been originally formed at a remote period, 

 since which all the many members of the class have been developed ; 

 and in order to discover the early transitional grades through which 

 the organ has passed, we should have to look to very ancient ances- 

 tral forms, long since become extinct. 



We should be extremely cautious in concluding that an organ 

 could not have been formed by transitional gradations of some kind. 

 Numerous cases could be given amongst the lower animals of the 

 same organ performing at the same time wholly distinct functions ; 

 thus in the larva of the dragon-fly and in the fish Cobites the 

 alimentary canal respires, digests, and excretes. In the Hydra, the 

 animal may be turned inside out, and the exterior surface will then 

 digest and the stomach respire. In such cases natural selection 

 might specialise, 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 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 

 branchise 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 indivi- 

 dual. 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, 



L 2 



