6o Evolution and Distribution of Fishes 



CHAPTER III. 



THE EVOLUTION OF FISHES FROM 

 INVERTEBRATES 



Numerous views have been advanced to account for 

 the origin of vertebrate or chordate animals from some 

 invertebrate stock. These are synopsized in various zoo- 

 logical textbooks, and need not now concern us. The 

 writer in his "Causes and Course of Organic Evolution" has 

 already adduced abundant reasons for accepting the group 

 Nemertinea as that which most exactly and most continuous- 

 ly satisfies requirements. Such a view had previously been 

 strongly advocated by Harting and especially by Hubrecht, 

 to some extent also by Balfour and Lankester. But all 

 of these authors dealt only with a few of the organs. In 

 this chapter the writer will review, and still further advance, 

 arguments and evidences in favor of this contention. 



The group Nemertinea as now existing, is made up of 

 genera and species, the simpler of which show surprisingly 

 graded connections with the Turbellaria, as pointed out 

 by several authors, and synopsized by Burger. In the 

 most evolved or metanemertean members of the Nemer- 

 tinea also these exhibit remarkable fundamental agreement 

 with the simpler chordate animals. All however are soft- 

 bodied, if we except the styliform teeth, and so far as yet 

 known have left no fossil remains. But as pointed out 

 in last chapter, their present geographical distribution com- 

 pels us to accept that they are a very ancient group. Their 

 distribution also, might on first thought cause us to conclude 

 that the group originated along littoral sea areas. For 

 nearly 450 of the 465 species now known, occur there. But 

 the wide extension of the nine freshwater and the eight 

 land species, also their continued persistence in spite of 

 denundation and geologic changes generally, strongly in- 

 cline us to favor a freshwater origin for the entire group. 



A very strong argument for this is, that in the rhab- 

 docoel turbellarians — the majority of which are fresh- 

 water — that important organ the proboscis develops as 



