228 THE EVOLUTION OF CONTINUITY 



thus be a factor in the production of bilateral symmetry. 

 The ballast would work for the maintenance of a habitual 

 poise, and tend to prevent rolling when the organism moved 

 through the water, and by its resistance to water-pressure 

 forces trying to produce a torpedo form would bring about 

 the compromise of streamline and bilateral symmetry. 



Assisting in the maintenance of an even keel in the majority of 

 known typical fishes (the exceptions being flat fish, marsipobranchs, 

 and elasmobranchs) there is a structure known as the swimming 

 bladder ; this lying beneath the vertebral column, and being a species 

 of sac whose contents are nitrogen in freshwater fishes, and oxygen 

 in those inhabiting salt water. Although in the Mudfishes (Dipnoi) 

 it appears to have evolved respiratory functions and to act as a facul- 

 tative lung, there can be little doubt that its presence in all other 

 cases is concerned with physical equilibrium. The gas the swimming 

 bladder contains seems to come from the fine bloodvessels ramifying 

 in its walls, and one is inclined to suppose that the organ was originally 

 produced as the direct result of water-pressure which caused the 

 escape of gas from the blood. The gas would naturally tend to collect 

 in the highest region of the fish, thus increasing the ballast effect of 

 the abdominal organs. On this supposition, the deeper the fish would 

 go the more gas would be in the bladder, and the nearer the surface 

 the reverse would be the case. 



The absence of a swimming bladder in flat fish, such as the flounder, 

 is interesting in relation to the body asymmetry exhibited. For the 

 flounder in its early days is bilaterally symmetrical, and swims on 

 an even keel after the manner of typical fish ; but as growth proceeds, 

 the animal generally takes to lying on one side on the sea-bottom, 

 finally adopting this pose permanently, and living and swimming 

 with one side uppermost. Thus, what appears to be the back or 

 dorsum of the flounder was originally the side. With the adoption 

 of the new position, one of the eyes finds itself applied to the sand 

 and temporarily useless, but it at once begins to work its way towards 

 the dorsal surface and comes to lie asymmetrically alongside of the 

 unaffected eye. Similarly, the mouth becomes twisted in adapting 

 itself to the new position. The body is also asymmetrical, the under 

 white surface being flatter than the dark dorsal one. 



It is safe to say that the developing flounder fell over because 

 it lost its previously maintained state of physical equilibrium, and 

 our suggestion is that this was at first preserved by the ballast of the 

 digestive tract. It might be that the falling over was the result of 

 relative decrease in weight of the visceral organs and contents owing 

 to the growing fish taking to a food of low density ; but it is also 

 interesting to note that the failure to develop a swimming bladder 

 could help to bring about the accident. On the other hand, elasmo- 

 branchs and marsupobranchs have not this organ, and they are not 

 flat fish. 



