26 



development of some of the systems, especially the nervous and genera- 

 tive? We will follow Cuvier, and the majority of ichthyologists, in pla- 

 cing next to each other the Dermopteri and Elasmobranchii. and then, in 

 ascending order, will succeed the Ganoids and finally the Teleostei. The 

 considerations which have led us to these results will be briefly stated. 



Fishes appear to be constructed according to four different sub-plans, 

 which are characterized by their correspondence to different stages or 

 grades of development of a typical osseous fish. The types so distin- 

 guished are equivalent to the four subclasses here admitted. All the sub- 

 classes are constructed on the ichthyic modification of the vertebrated 

 plan, but the archetype of each subclass corresponds, in a general man- 

 ner, to different periods of development of that plan which have success- 

 ively taken place. 



A plan will admit of great modification or adaptation. Species con- 

 structed according to an inferior plan do not necessarily exhibit complex- 

 ity of organization of less degree than those constructed on a higher 

 plan. Species framed on inferior plans are capable of a high degree of or- 

 ganization consistent with the plan. The degree of development of the 

 plan or type is the criterion of the station of a group ; not the complexity 

 of organization, or adaptation to peculiar habits or modes of life. Ap- 

 plying these principles to the relative classification of the different sub- 

 classes of Fishes, we have the following results. 



The Dermopteri are the lowest in the scale ; they represent an early 

 stage of development of the archetype fish. The subclass exhibits no 

 examples of complexity of organization, or specialization and develop- 

 ment of any system or set of organs. 



The type of structure of the Elasmobranchii is more perfected, and 

 represents the development of the archetype fish ai-rested at a more ad- 

 vanced period. It still exhibits many embryonic characteristics, but on 

 account of the complexity and high degree of development and adaptation 

 of special system > or sets of organs, exhibits a high upward tendency. 

 The subclass of the Ganoids represents a still more advanced period of 

 development, which is more remote from the germ of the embryo than 

 that of either of the preceding. Representatives of this subclass also 

 exhibit a high degree of special development of different systems, but this 

 is of subordinate value to the plan or type of structure. 



Finally, the sub-plan of the Teleostei represents the perfected idea of 

 the ichthyic development of the vertebrated plan. For the sake of illus- 

 tration, it may be said to exhibit the highest advance consistent with the 

 preservation of the ichthyic type. 



The conception of the sub-plans and the range of variation exhibited in 

 each, may be represented by four vertical lines. 



The Dermopteri are to be represented by a short line. 

 The Elasmobranchii may be represented by a much elongated line, 

 commencing at a higher level than the line for the Dermopteri. 



The Ganoidei also exhibit a high upward tendency, and an approxima- 



