DIFFICULTIES ON THEORY 171 



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 alimentary 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 swim bladder 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 view is now generally held, a 

 part of the auditory apparatus has been worked in as a 

 complement to the swimbladder. All physiologists 

 admit that the swimbladder is homologous, or ' ideally 

 similar' in position and structure with the lungs 

 of the higher vertebrate animals : hence there 

 seems to me to be no great difficulty in believ- 

 ing that natural selection has actually converted a 



