No. 1 8.] 



TRIASSIC FISHES OF CONNECTICUT. 



13 



ideas — those which include the principle of continuity and the 

 higher principle of development — became effectual in the world 

 of thought, and enlightened mankind through the medium of 

 Darwin's utterances. Both in history and in natural science 

 the spirit of investigation was completely transformed by these 

 ideas. 



So much by way of parenthesis. Enough has now been said 

 to show that the natural history of the past possesses rich interest 

 from a culturo-historical or humanistic standpoint. And from 

 these matters we pass on to a consideration of our special theme, 

 the study of fossil fishes in general. Before speaking, however, 

 of the introduction and succession of the class of fishes in point 

 of chronological sequence, it may be useful to insert at this 

 point a table showing the principal divisions of the stratigraphic 

 column, which will serve the double purpose of indicating the 

 position of our Triassic rocks in the system, and marking the 

 advent of successively higher classes of vertebrates. Forma- 

 tional units, it is to be observed, are divided into systems, series, 

 and groups; and the corresponding time-relations are expressed 

 by the terms eras (or ages), periods, and epochs. Following 

 is the commonly accepted arrangement: 



GEOLOGICAL TIME-SCALE. 



