POISSONS 



t97 



Queue Rhipidicerque 

 Dorso-Anale 



/ Vie Nectique \ Evolution Progressive daire 



\ Secondaire / de la Natation 



Queue Géphyrocerque 

 Homocerque 

 * 



Queue Rhipidicerque 

 Homocerque 



/ \ 



\ ie Benthique 



Evolution Régressive 

 de la Natatii >n 



Queue Rhipidicerque 

 Hétérocerque 



Vie Nectique 

 Primaire 



Evolution Progressive Primaire 

 de la Natation 



( )ueue Diphycerque 



En d'autres termes, un cas tout à fait parallèle à celui de Cyema atrum parmi les 

 Physostomes. 



Et un nouvel, et magnifique, exemple de l'Irréversibilité de l'Evolution ! (') 



C'est ce que M. G. A. Boulenger exprime ainsi : « The diphycercal or isocercal termi- 

 nation of the vertébral column. This has often been regardée! as a primitive character ; but if 

 we accept, as I do, the conclusions of Dollo in bis remarkable discussion of the Dipneusti, \ve 

 cannot hesitate to lay clown as an axiom that ail Teleosteans are originally descended from 

 heterocercal forms. But the caudal fin may become reduced or disappear, as in the séries 

 Morm yrops-Gymnarchus, Urenchelys-Murœna , Thyrsites-Trichiunis, Pleur omet es-Cynoglossus, to men- 

 tion only examples in which the direction of the Une of évolution dues not seem open to 

 controversy ; and if it should reappear, it cannot be again in the specially modified condition 

 known as homocerev. ».... « I hâve reason to believe that the Gadoids must hâve been derived 

 from such a group as the Beryciche, through forms of which the Macruridae, with thoracic 

 ventral fins composed of 7 to 12 rays, are the nearest known examples, and in which the caudal 

 fin had entirely vanished. I regard the isocercal condition of the Gadidse as the resuit of the 

 formation of a new caudal fin, the homocercal extremity of the vertébral column having 1 een 

 lost by the direct ancestors ot thèse fishes ( z ). » 



« Anacanthini.... Caudal, if présent, without expanded hypural, perfectly symmetrical, 

 and supportée! by the neural and haemal spines of the posterior vertebrae and by basai 1 

 similar to those supporting the dorsal and anal rays. This type of caudal fin must 1 ded, 



as I hâve pointed out, as secondary, the Gadidse being no doubt derived from fishes like the 

 Macruridae, in which the homocercal fin had been lost (°). » 



(1) L. Dollo. Lois de l'Évolution, etc. p. i65. 



(2) G. A. Boulenger. Notes on the Classification of Teleostean Fishes. 

 of the Pleuronectidœ (Ami. Mag. Nat. Hist. 1902. Vol. X. p. 298). 



(3) G. A. Boulenger. A Synopsis, etc. p. 17''. 



IV. < Mi the Systematic I 



