324 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



IV. THE LINE OF THE PISCES. 



The fishes form various series and subseries, and the tracing 

 of all of them is not yet practicable, owing to the deficiency in 

 our knowledge of the earliest or ancestral forms. Thus the ori- 

 gins of the four subclasses, Holocephali, Dipnoi, Elasmobranchii, 

 and Teleostomi, are lost in the obscurity of the early Palaeozoic 

 ages. 



A comparison of the four subclasses just named shows that 

 they are related in pairs. The Holocephali and Dipnoi have no 

 distinct suspensory segment for the lower jaw, while the. Elasmo- 

 branchii and Hyopomata have such a separate element. The lat- 

 ter, therefore, present one step in the direction of complication be- 

 yond the former ; but whether the one type is descended from the 

 other, or whether both came from a common ancestor or not, is 

 unknown. If one type be derived from the other, it is not certain 

 which is ancestor, and whether the process has been one of advance 

 or retrogression. The fauna of the Permian eijoch throws some 

 light on the relations of these subclasses in other respects. The 

 order of the Ichthyotomi,* while belonging technically to the 

 Elasmobranchi, presents characters of both the Dipnoi and the 

 Teleostomi. It is so near to the Dipnoi in the characters of the 

 skull that nothing save the presence of a free suspensor of the 

 lower jaw prevents its entering that subclass. It indicates that 

 the one of these divisions is descended from the other, or both 

 from a common division which may well be the group Ichthyotomi 

 itself. In case the Elasmobranchi have descended from the Ich- 

 thyotomi, they have undergone degeneracy, as the Ichthyotomi 

 have a higher degree of ossification and differentiation of the bones 

 of the skull. If they descended from a purely cartilaginous type 

 of Dipnoi, they have advanced, in the addition of the free hyo- 

 mandibular. If the Dipnoi have descended from either division, 

 they have retrograded, in the loss of the free hyomandibular. As 

 regards the Teleostomi, we have a clear advance over the other 

 subclasses in the presence of the maxillary arch and the opercular 

 apparatus. 



Too little is known of the history of the subclasses, excepting 

 the Teleostomi, for us to be able to say much of the direction of 



* See " Palaeontological Bulletin," No. 38, E. D. Cope, 1884, p. 5*72, on the genus 

 Didymodus. 



