326 THE STRUCTUKAL EVIDENCE OF EYOLUTION. 



supporting elements of the pectoral and ventral fins by the reduc- 

 tion of their numbers and length. According to paleontological 

 history, however, the tribe of Teleostomi with most contracted 

 fins, Actinopteri, appeared in the Coal Measures (Paleoniscidse), 

 or very soon after the Crossopterygia in the Devonian. 



The descent of the fishes in general has witnessed, then, a 

 contraction of the limbs to a very small compass, and their sub- 

 stitution by a system of accessory radii. This has been an ever- 

 widening divergence from the type of the higher Vertebrata, and 

 from this standpoint, and also a view of the ^Hoss of parts with- 

 out complementary addition of other parts," may be regarded as 

 a process of degradation. 



Taking up the great division of the Actinopteri, which em- 

 braces most of the species of living fishes, we can trace the direc- 

 tion of descent largely by reference to their systematic relations 

 when we have no fossils to guide us. 



The three sub tribes adopted by Jordan represent three series 

 of the true fishes which indicate lines of descent. The Holostei 

 include the remainder of the old ganoids after the subtraction of 

 the Crossopterygia and the Chondrostei. They resemble these 

 forms in the muscular bulbus arteriosus of the heart and in the 

 chiasm of the optic nerves. Both of these characters are com- 

 plexities which the two other divisions do not possess, and which, 

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

 and therefore to that extent degenerate. Of these divisions the 

 Physostomi approach nearest the Holostei, and are indeed not 

 distinctly definable without exceptions. The third division, or 

 Physoclysti, shows a marked advance beyond the others in : (1) 

 the obliteration of the primitive trachea, or ductus pneumaticus, 

 which connects the swim-bladder and oesophagus ; (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 presence of numerous spinous rays in the fins ; and (5) the 

 roughening of the edges of the scales, forming the ctenoid type. 

 There are more or less numerous exceptions to all of these char- 

 acters. The changes are all further divergencies from the other 

 vertebrate classes, or away from the general line of ascent of the 

 vertebrate series taken as a whole. The end gained is specializa- 

 tion ; but whether the series can be called either distinctively pro- 

 gressive or retrogressive, is not so clear. The development of 

 osseous spines, rough scales, and other weai)ons of defense, together 



