Evolution of Animals 405 



fishes attain that enormous marine development which now 

 characterizes them; so that, while 133 genera of recently evolved 

 living teleosteans are fresh-water, 747 genera are marine. This 

 remarkable development seems to have been correlated with 

 a gradual dying out alike of the dominant groups of cepha- 

 lopod molluscs and of selachean fishes. So, when we sum up 

 all of the living genera of gnathostome fishes and find that 

 453 are fresh-water and 902 are marine, this by no means 

 represents the primitive distributional relation. 



We would therefore consider that all palseontological, struc- 

 tural, and geographical evidence strongly favors the view that 

 the primitive fishes have had a fresh- water origin and that 

 derivative marine forms have evolved from these, which, in 

 the case of the anciently derived selacheans and of the recent 

 teleosteans, reached each to a high climax of species diversity 

 — the former during the cretaceous period, the latter in our 

 own day. 



The Batrachia or Amphibia are and have been so evidently 

 inhabitants of rivers, ponds, or land areas that an enumeration 

 of the genera might seem to suflace for our present purpose. 

 Boulenger concludes "that the geographical distribution of 

 the Amphibia agrees in general with that of the fresh-water 

 fishes" (Camb. Nat. Hist. VIII (1909) 69). But the haunting 

 notion of a marine — at least a shore origin — for the main ver- 

 tebrate lines — seems to pervade so much the minds and the 

 volumes of most students that it may be well here to emphasize 

 — what is expanded on succeeding pages — -namely that from 

 a fresh-water nemertinean ancestry we would derive the 

 main vertebrate line through a fresh-water cyclostomatous 

 type to a coecilian and thence to a urodelan stage. On the 

 other hand the derivation of the varied carboniferous and 

 permian cuirassed batrachians from primitive fresh-water 

 ganoid progenitors, or from more ancient early silurian or 

 Cambrian forms ancestral to both, seems at least possible, 

 but the question is open to discussion. Of the nearly 2200 

 species that fall under 171 genera, that now make up the 

 Batrachia, all may truly be reckoned as land or fresh-water, 



