DEVONIAN FISHES OF IOWA 51 



III. 



Evolutionary History of Fishes and Scheme of Their 

 Systematic Arrangement. 



The geological succession of the class of Fishes is very sat- 

 isfactorily known from the middle of the Silurian onward, and 

 the essential features of the past history of this chain of life 

 are now comparatively well ascertained. Facts have been 

 brought to light, and broad generalizations established upon 

 them, through study of this history, which have an important 

 bearing upon many of the fundamental problems of biology, 

 illustrating as they do not only general principles of organic 

 evolution, but enabling us to construct a natural system of classi- 

 fication reflecting the development, and expressive of the true 

 relationships of Pisces. As might be expected, the evolutionary 

 history of Fishes is consistent at all points with those principles 

 of organic development that are the final outcome of investiga- 

 tion carried on in other departments of palaeontology ; and in- 

 deed, it is within the very group we are now considering that 

 some of the fundamental laws of organic progress have been 

 discovered. 



It will suffice for our present purpose to summarize briefly the 

 main facts concerning the geological succession observed among 

 Fishes, and thereafter to explain the general basis of classifica- 

 tion that is now currently adopted. And in the first place we 

 must note that concerning the immediate origin of the group of 

 Fishes and fish-like vertebrates, palaeontology reveals no certain 

 clue. Nor is it likely from the nature of things that authentic 

 documents will ever be discovered, there being abundant reason 

 to suppose that ' the primitive forerunners of the vertebrate 

 phylum were soft-bodied creatures, and incapable of preserva- 

 tion in the rocks. 



As to the once popular theory which still finds adherents in 

 some quarters, namely, that the earliest fish-like vertebrates are 

 derived from the Arthropod stem, sharing features in common 



