Pahfiontohgij lo Bivlogy. 315 



Now fi-slies are uquatic animals, and nearly all the fo.ssili- 

 fcrons rocks were deposited in water. The past history of 

 this chain of lite ou^lit therefore to Ijc almost conij)letely 

 revealed by the geological records. ]\Iaking due allowance 

 for the imperfection of collections and the accidental nature 

 of the discovery of fossils, the general outlines of this history 

 may indeed be considered as tolerably well ascertained. Thus 

 the facts of Paleontology not ordy aid the biologist in 

 discovering the true relationships of the fishes ; at the same 

 time they afford a definite means of determining with certainty 

 some of the fundamental principles of organic evolution 

 illustrated by them. As identical principles may be deduced 

 from other departments of Pakeontology, most of them are 

 }iot likely to be altered in any essential respects by future 

 discoveries. 



It must suffice here to allude only to a few of these general 

 results which seem to be of far-reaching importance, omitting 

 details which may be obtained from special treatises. Fore- 

 most among them is the demonstration that the evolution of 

 the animal world has not proceeded uniformly, but in a 

 rhythmic manner. As soon as fishes had acquired the 

 })addle-shapetl paired fins, they suddeidy became the special 

 leature of the Devonian period in all parts of the globe that 

 have hitherto been geologically examined, and they attained 

 their maximum development, being more numerous and more 

 diverse in form than at any subsequent time. None of these 

 paddle-finned fishes [Crossopterygii) in the course of their 

 varied development made much approach towards passing 

 into the next grade of fish-life with short-based paired fins 

 and a heterocercal tail [Chondrostei) ; but among their 

 earliest representatives there was at least one member of the 

 higher group, which suggests that the latter arose when the 

 previous group was just becoming vigorous. At the be- 

 ginning of the Carboniferous period the higher grade of fish- 

 life just mentioned suddenly became the dominant feature, 

 and during the Carboniferous and Permian it attained its 

 maximum development. Towards the close of the Permian 

 period the next higher group was heralded by only one 

 representative, but as soon as it arose in the Trias it resembled 

 its predecessors in becoming immediately dominant, sur- 

 passing all contemporary races of fishes both in the number 

 of individuals and in the variety of genera and species. In 

 the Cretaceous period the highest bony fishes appeared, and 

 at the end of that period, with the dawn of the Tertiary, they 

 suddenly diverged into nearly all the subdivisions which 

 characterize the existing fish-fauna, accomplishing much more 



