I02 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



two former characters are complexities which the two 

 other divisions do not possess, and which, as descend- 

 ants coming later in time, must be regarded as inferior, 

 and therefore to that extent degenerate. Of these di- 

 visions the Malacopterygia approach nearest the Ho- 

 lostei, and are indeed not distinctly definable without 

 exceptions. The third division, or Acanthopterygia, 

 shows a marked advance beyond the others in : (i) the 

 obliteration of the primitive trachea, or ductus pneu- 

 maticus, which connects the swim-bladder and ceso- 

 phagus ; (2) the advance of the ventral fins from the 

 abdomen forward to the throat ; (3) the separation of 

 the parietal bones by the supraoccipital ; (4) the pres- 

 ence of numerous spinous rays in the fins ; and (5) the 

 roughening of the edges of the scales, forming the cten- 

 oid type. There are more or less numerous excep- 

 tions to all of these characters. The changes are all 

 further divergencies from the other vertebrate classes, 

 or away from the general line of ascent of the verte- 

 brate series taken as a whole. The end gained is spe- 

 cialization ; but whether the series can be called either 

 distinctively progressive or retrogressive, is not so 

 clear. The development of osseous spines, rough 

 scales, and other weapons of defense, together with 

 the generally superior energy and tone which prevail 

 among the Acanthopterygia, characterize them as su- 

 perior to the Malacopterygia, but their departure from 

 the ascending line of the Vertebrata has another ap- 

 pearance. 



The descent of the Acanthopterygian fishes has 

 probably been from Holostean ancestors, both with 

 and without the intervention of Malacopterygian forms. 

 This is indicated by increase in the number of basilar 



