256 Evolution and Distribution of P'ishes 



CHAPTER IX. 



Fishes in Time and Space. ( i ) The Primitive Fishes. 



In this and several succeeding chapters, the writer pro- 

 poses to trace, as perfectly as possible, the gradual evolution 

 of the groups of fishes, the probable relation of these to 

 each other, and the distributional areas that each group 

 spread over, or is known to have occupied. 



In previous chapters we have endeavored to show that 

 the entire group of fishes — and so of the vertebrates as 

 a necessary corollary — originated as direct and progressive 

 derivatives from higher types of metanemerteans, which 

 flourished during Cambro-Ordovician times. And as is 

 demonstrated by a few of these, that still linger on as 

 surviving remnants from primitive epochs, they early 

 showed a tendency toward evolution along at least two 

 lines which when pursued further gave rise to at least two 

 diverging groups. These both inherited common and 

 fundamental characters derived from the Nemerteans, but 

 they also showed steadily acquired and progressively evolv- 

 ed details in diverging lines, that were superadded to the 

 nemertean structure. These divergent details represented 

 slowly acquired characters, that more and more distinguish- 

 ed the two diverging groups. 



Putting aside the Urochordata and the Cephalochor- 

 data that we have shortly discussed elsewhere (/:539), 

 and reference to which is constantly made in sections of 

 this work, the following questions naturally arise: (a) what 

 are the most primitive fishes as yet recognized; (b) in what 

 region may they first have been evolved; (c) how widely 

 did they become distributed; (d) how long did they persist? 

 In any attempted answer the extreme incompleteness of 

 the geological record, and also the comparatively soft 

 nature of these primitive types, except where a calcareous 

 skeletal system evolved, must ever be kept in view. The 

 graphic table opposite, which will be analyzed and explained 

 later from the structural standpoint, indicates the succes- 

 sive evolutionary lines that the writer proposes to follow. 



