RECAPITULATION. 495 



and certain other Scombresocidce are analogous. The tail 

 is homocercal, but the ventral lobe is distinctly larger and 

 stiffer than the dorsal. Physiologically therefore the tail 

 acts like that of Ichthyosaurus. Here however the counter- 

 action cannot be produced by the pectorals, because they 

 are too long and too weak, and moreover are observed to 

 be folded against the sides of the body in rapid swimming. 

 The necessary effect is produced by the pelvic fins, the 

 specific gravity of the flying fish being less than that of the 

 water. 



Ahlborn takes a somewhat different view. He con- 

 siders that there are important advantages in the action of 

 heterocercal tails which Schultze has not perceived. In his 

 opinion the bottom fish requires to have a tail which will 

 rise in order to prevent it rubbing against the ground in its 

 stroke and therefore failing to propel the fish when the latter 

 desires to put itself in motion. In the case of the surface 

 animal, Ichthyosaur or flying-fish, a symmetrical tail might 

 when put into motion at the surface pass into the air, and 

 so the creature would become for the moment helpless ; it 

 therefore requires a tail which will tend to pass more deeply 

 into the water. He calls the ordinary heterocercal tail epi- 

 batic, the symmetrical tail isobatic, and the tail with reversed 

 heterocercy hypobatic. 



It is clear that none of these considerations help us to 

 understand how on Lamarckian principles the various forms 

 of tail arose, and even with regard to function it seems to 

 me that the two authors mentioned have overlooked some 

 of the most important points. It seems to me that the 

 chief use of a heterocercal tail to a fish that lives near the 

 bottom is the very fact that such a tail throws the hinder 

 end of the body up and keeps its nose on the ground, for 

 it is on the ground that it finds it food. It is a remarkable 

 fact that the heterocercal tail is usually associated with a 

 snout projecting beyond the mouth, and such a snout is in 

 a large number of cases, if not in all, used for routing in 

 the ground in the search for food. Such is certainly the 

 case in the sturgeon, and it is true of some at least of the 

 Selachians ; Scylliuvi cauicula and catulus feed on lower 



