CLASSIFICA TION y 



enormous structural and physiological changes had of 

 necessity to have been attained. The frame of the 

 head and trunk has become moulded as in the fish's 

 form, contours have been elaborately filled out and 

 rounded, median dermal keels developed, vein valves lost, 

 and the legs transformed into fin-like appendages. 



The form of the fish is accordingly to be looked upon as 

 cast in a more or less common mould by its environment. 

 Its internal structures, as in the cetacean, are also ob- 

 served to be modified in accordance with its external form. 

 This is a factor in the evolution of fishes which appears 

 in every group and sub-group. And it has ever stood in 

 the way of classifying them satisfactorily according to 

 their kinships. 



"Fishes," used as a popular term, may include Lam- 

 preys, Sharks, Chimaeroids, Lung-fishes, and " Modern 

 Fishes" (Teleostomes), — the major groups to be dis- 

 cussed in the present book. But the relative position of 

 each of these divisions must at present remain more or 

 less doubtful. The group of the Lampreys is certainly 

 widely removed from the remaining ones, standing mid- 

 way between the simplest chordate, Amphioxus, and the 

 true fishes : it is usually given a rank co-ordinate with 

 either of these, and, in fact, with all other groups 

 of vertebrates, taken collectively. Sharks, Chimaeroids, 

 Teleostomes, may be taken to represent true fishes ; and 

 each might be assigned co-ordinate rank, although geneti- 

 cally the Chimaeroids are certainly far more closely allied 

 to the Sharks than are the Teleostomes. The Lung-fishes, 

 as a widely divergent group, appear, as W. N. Parker has 

 suggested, to be reasonably entitled to a rank equivalent 

 to that of the three groups of true fishes taken together. 



