84 EVOLUTION OF TO-DAY. 



mentary beginning. To use once more the above 

 illustration : the descent theory would say that the 

 air-bladder of fishes is not a useless organ, being of 

 importance as a hydrostatic apparatus, and its pres- 

 ence in the fish is explained, not as a prophecy of 

 coming lungs, but as being of direct use to the ani- 

 mal. The theory would go on to say, that this air- 

 bladder in certain fishes in the past became more 

 complicated, and that in some cases there was de- 

 veloped within its walls a considerable supply of 

 blood-vessels, so that by taking air into this air- 

 bladder the fish could carry on a certain amount of 

 its respiration. And this is not hypothesis simply, 

 for some fishes (Dipnoi) do make use of swallowed 

 air in this way. Now these fishes, the theory goes 

 on to say, being able to respire slightly by means of 

 this highly developed air-bladder, would be able to 

 survive if left upon the beach for a short time by 

 the retreating tide. Perhaps they would acquire the 

 habit of escaping from their enemies by retreating 

 into shallow water when the fall of the tide would 

 leave them for some time exposed to the air. Such 

 favored individuals would survive and multiply, and 

 the habit of breathing air would become more 

 marked. Finally, the air-bladder would become so 

 well developed by use, that its possessor could live 

 for a long time out of water, breathing air by its 

 means, or could still live in the water like a fish : a 

 condition found in the amphibia. A true land ani- 

 mal would thus arise breathing by means of lungs. 

 Meantime other fishes whose air-bladders did not 

 present these favorable variations, or who did not 



