138 licconls of the Indian Museuni. [Vol. XV, 



upward, causing a bend in the body convex to the bottom, would be 

 accompanied by a hish of the tail ; it would be an advantage to have 

 the propelling force more or less in line with the anterior end of the body, 

 and thus the ventral side of the caudal fin would have a greater demand 

 made upon it than the dorsal. 



If the fish is represented as rising from the bottom by the bent arrow 

 in the diagram (iiu. 2), the broken line will indicate the posterior conti- 

 nuation of the line of progress after the tail has completed a lash ; the 

 dotted lines would represent the form of fin useful for this purpose. 

 This is merely a suggestion put forward until a better one is forth- 

 coming, but correct or no mechanically, certain it is that all fishes with 

 heterocercal tendencies developed the lower lobe of the caudal fin and 

 never the upper. 



Fig. 2. 



Having discussed the probable stimulus effecting heterocercy, there 

 still remains the question as to why it was at all necessary to develop 

 this asymmetry. When we remember that heterocercy is but a stage in 

 the production of homocercy, it is clear that external symmetry was the 

 ideal to be reached ; why then was the symmetry of protocercy aban- 

 doned only to be again attained ? Again the answer is one of mechanics ; 

 had the protocercal form been merely expanded, the internal skeletal 

 supports of this fin could not have met the demand made upon them. 

 The epural and hypural elements would have been long and practically 

 parallel with the axis ; this in itself would not have been mechanically 

 strong. Moreover, the centre of the caudal fin would have been weak, 

 there being no direct support for the fin-rays in the middle line, and it 

 is here that streiigth is most needed. An essential, therefore, for a 

 strong caudal fin is a firm support for the dermotrichia forming the 

 greater part of the fin especially near the centre. 



It is in the final product of homocercy that one must look for an 

 interpretation of the meaning of heterocercy. The examination of such 

 a caudal fin as that of Scoiiiber, well known as one of the fastest of swim- 

 mers, shews how the result has been attained ; the upturned axis pro- 

 vides the supports of the fin-rays with a firm attachment, and the sup- 

 ports are seen to radiate from a centre which is in line with that of the 

 axis. In the majority of Teleosts the hypurals, which always support 

 the greater number, and sometimes even the whole of the caudal fin- 

 rays, are firmly fused to the vertebral elements. It will therefore be 

 seen that the reason for the heterocercal stage is that the ventral fin- 

 ray supports may be brought into the same line as that of the axis, and 

 at the same time afford a strong attachment for them. It was the only 

 way these ends could be achieved ; symmetry had to be abandoned 



