RECAPITULA TION. 493 



Dipnoi. In other cases as in the Sturgeon and many other 

 Ganoids the hypertrophy of the ventral lobe produces the 

 heterocercal condition. Finally, in Teleosteans, the upper 

 lobe or original terminal portion of the tail is aborted 

 altogether, and the vental lobe becomes terminal and 

 acquires a secondary symmetry. It is unnecessary on the 

 present occasion to trace the extent of recapitulation in all 

 the various cases. It will be sufficient to refer to the 

 homocercal Teleosteans, for example, the Pleuronectidae, 

 Clupeidas, or Salmonidse. In these the larva has at first 

 a symmetrical membranous tail without fin-rays. In this 

 minute radiate fibres appear, which clearly corerspond to 

 the horny fibres of Elasmobranchs and Ganoids. The 

 cartilaginous rays are but little developed in the fin itself, 

 but the basal portions within the trunk, that is the epural 

 and hypural cartilages are developed and soon replaced by 

 bone. When the dermal fin-rays appear they are from 

 the first developed on the ventral side behind the termina- 

 tion, and the terminal portion never develops them. This 

 terminal portion is bent up by the hypertrophy of the 

 ventral lobe and becomes rudimentary. 



Thus in these fishes the tail is at first diphycercal, then 

 in development passes through a heterocercal condition and 

 afterwards acquires the secondary symmetry called the 

 homocercal condition. 



Two zoological thinkers have recently made endeavours 

 to ascertain the advantages of the various forms of fishes' 

 tails in adaptation to different circumstances. They do not 

 refer to the origin of the different forms, but to their mechan- 

 ical, or adaptational value. This in itself is of great im- 

 portance and interest, and to the zoologist who thinks in 

 the terms of selection the use or advantage of each form 

 appears to be the explanation of its evolution. F. E. 

 Schultze develops his views in connection with a con- 

 sideration of the tails of the Ichthyosaurians which are 

 heterocercal, but with an asymmetry which is the reverse of 

 that seen in fishes. The terminal portion of the vertebral 

 column in these reptiles was bent downwards, and on the 

 upper side of it was a somewhat triangular cutaneous fin, in 



