RELATIONS TO OTHER SCIENCES 561 



of this history may indeed be considered as tolerably well ascertained. 

 Thus the facts of paleontology not only aid the biologist in discov- 

 ering 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 paleontology, most of them are not 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. Foremost among them is the 

 demonstration that the evolution of the animal world has not pro- 

 ceeded uniformly but in a rhythmic manner. As soon as fishes had 

 acquired the paddle-shaped paired fins, they suddenly became the 

 special feature 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 beginning of the Carbon- 

 iferous 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, surpassing all con- 

 temporary 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 

 evolution in a brief interval than has taken place during the whole of 

 the succeeding Tertiary time. In short, the fundamental advances 

 in the grade of fish-life have always been sudden and begun with ex- 

 cessive vigor at the end of a long period of apparent stagnation; while 

 each advance has been marked by the fixed and definite acquisition 

 of some new character an "expression point," as Cope termed it 

 which seems to have rendered possible, or at least been an essential 

 accompaniment of, a fresh outburst of developmental energy. As we 

 have seen, the successive "expression points" among fishes were the 

 acquisition of (1) paddle-like paired fins, (2) shortened fin-bases but 



