HOW THE FISHES LEAENED TO SWIM HEINTZ 245 



The next group, the Holostei (fig. 11, D; fig. 12, C), which abound 

 in the Mesozoic, have still to a large extent preserved their ganoid 

 scales, but the internal skeleton is much more strongly ossified and 

 the tail has become almost entirely homocercal (evenlobed) as the 

 scale-covered body axis in the upper lobe is strongly reduced. 



Finally the recent Teleostei (fig. 11, E; fig. 12, D) have developed 

 a completely ossified skeleton, and their scales are very thin and 

 light, sometimes even completely absent, and their tails have become 

 entirely homocercal. In these forms we therefore have the most 

 perfect type of swimmers (fig. 2, A). 



The evolutionary series here described is, of course, entirely dia- 

 grammatic. One cannot, of course, consider, for instance, the Ostra- 

 coderms or Placoderms as real and direct ancestors of the younger 

 fishes. 



But, as in numerous other series of comparable though not directly 

 related evolutionary types, this series shows how the evolution in 

 all probability has progressed, and how the different fundamental 

 types have superseded each other. In general the evolution has 

 proceeded parallel and independently within numerous differ- 

 ent groups. In our schematic representation we therefore only bring 

 together our examples of the different steps of evolution from the 

 different lines which frequently developed quite independently of 

 each other although on the same fundamental pattern. 



