Fig. 7.— The Evolution of Fins. A. B. represent ideal shark-like 

 primitive fishes. In A. there are no separate fins, only continuous 

 folds of membranes. In B. separate fins have been derived by the 

 disappearance of certain portions of the once continuous membrane. 

 C. D. show how the paired fins gradually increased in complexity, 

 separate bars of cartilage in the earliest type of fin ( C. ) have gradu- 

 ally grafted themselves on to one simple bar which form an axis as 

 in D. E. F. show the difference between the fin which has three 

 distinct articular elements at its base (£.), and that which has but 

 one {F.). C. H. show the difference between the fringed fin ((?.) 

 and the fan fin (£?.). 



