34 Evolution and Distribution of Fishes 



Furthermore, the passage Inland of such fishes is for the 

 performance of that most important and primary life- 

 function — the reproduction of the species. All known evi- 

 dence indicates, that the inland situation sought out for the 

 spawning act represents a habit of ancient and hereditary 

 character, which antedates the period when such animals 

 tended to pass from a freshwater to a marine life. 



V. Terrestrial oscillations and stratigraphic successions. 



In the development of geology as a science during the 

 past century of effort, thick rock sections have often been 

 hurriedly examined and charted; fossils found in some 

 stratum of the section have been mixed with others from 

 higher or lower strata; inaccurate statements have often 

 subsequently been made as to the exact zone where the 

 fossils were found; while not unfrequently a single and *at 

 times even thin stratum with its organic contents, has been 

 described as typical of the entire section. But during the 

 past thirty years, as more exact methods of observation 

 and recording have been introduced, every changing type of 

 rock is duly described and measured even though only a 

 half inch to inch in thickness. In studying these on their 

 charts then, one is often surprised or at times even puzzled, 

 by the apparent suddenness and sharpness of the transition 

 from one palaeontological facies to another, and not un- 

 frequently also from one to another and different type of 

 rock. These are indications of the frequent oscillations that 

 have occurred in the earth's crust, and of the suddenness 

 with which one congeries of organisms may have been 

 blotted out, and another ushered in, that may have differed 

 markedly in ecologic and taxonomic affinities from the 

 former. Neglect to correlate, in exact stratigraphic succes- 

 sion, the organisms that are typical for each stratum passed 

 through, has been the main and a fertile source of erroneous 

 opinions in the past. 



On the other hand, the continuous persistence over wide 

 areas of Europe and North America of thin strata, that 

 possess a uniform lithological character, and that enclose 

 fossils of highly typical nature, often furnishes valuable 

 means for determination of the successional relations of 



