190 DIFFICULTIES ON THEORY. Chap. VI. 



passed, we should have to look to very ancient ancestral 

 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 the ali- 

 mentary canal respires, digests, and excretes in the larva 

 of the dragon-fly and in the fish Cobites. 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 easily specialise, if 

 any advantage were thus gained, a part or organ, which 

 had performed two functions, for one function alone, 

 and thus wholly change its nature by insensible steps. 

 Two distinct organs sometimes perform simultaneously 

 the same function in the same individual ; 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 having a ductus pneumaticus for its supply, 

 and being divided by highly vascular partitions. In 

 these cases, one of the two organs might with ease be 

 modified and perfected so as to perform all the work 

 by itself, being aided during the process of modification 

 by the other organ; and then this other organ might 

 be modified for some other and quite distinct purpose, 

 or be quite 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 one for a wholly 

 different purpose, namely respiration. The swimbladder 

 has, also, been worked in as an accessory to the auditory 

 organs of certain fish, or, for I do not know which 



