54 THE STORY OF FISH LIFE. 



are represented, beginning with the diphycercal 

 stage 1 passing through the heterocercal 

 stages, 2, 3, 4 and ending in the homocercal 

 stage 6. But our story must not end here. We 

 naturally ask : Is there any explanation for this 

 series of phases? Is any lesson to be derived 

 from these facts ? Well, in the first place, it 

 cannot be denied that we have, in studying these 

 phases, an admirable illustration of the evolution 

 of a tail. Here, in the flounder, we have watched 

 its gradual transformation from a perfectly sym- 

 metrical organ, through an asymmetrical, and 

 back to an apparently symmetrical form again. 

 We say apparently advisedly, for this last stage 

 is but a superficial symmetry, brought about by 

 the excessive development of one part at the 

 expense of the other. But what gain can the 

 result of this series of evolutional phases be to 

 the fish ? Or, rather, if these changes be for an 

 ultimate end, why cannot this be attained at 

 once, without the transformations 1 



The gain to the fish is possibly a double one. 

 In the first place, in passing from the diphycercal 

 to the heterocercal tail, the vertebral column is 

 shortened, and hence there is effected at once a 

 saving of material, and a greater rigidity and 

 power added to the tail. So much for the gain. 

 But why, it may be asked, cannot this gain be 

 effected at once 1 Why this need for circum- 

 locution ? The answers to these questions are 

 not easily set down in a few words ; but, never- 

 theless, answers most satisfactory answers are 

 forthcoming. Moreover, they not only set at 

 rest these questions, but at the same time they 



