24 Evolution and Distribution of Fishes 



If we confine attention to the freshwater genus Sticho- 

 stemma, the seven known species occur In Pennsylvania, 

 Britain, France, Germany, Austria, Switzerland, Russia, 

 Turkestan and East Africa. The terrestrial genus Geo- 

 nemertes Includes seven species distributed over the Ber- 

 mudas, Rodriguez, the Palau Isles, New Zealand, Aus- 

 tralia, New Guinea, and Samaral Isle. The littoral marine 

 species are world-wide, and even such genera as Prostoma 

 {Tetrastemma) with 63 species, Cerebratulus with sixty- 

 three species, and Amphiporus with sixty-three species are 

 all represented by some species over every littoral region. 

 The marine genus Tiibtilanus Is represented by species 

 from the frozen seas of Greenland southward along the 

 Pacific and Atlantic coasts to the tip of South America. 



Such considerations at once cause us to Inquire whether 

 the earth has undergone fundamental changes — alike of 

 elevation and depression — in the past; whether the main 

 landmasses have — as claimed by the earlier geologists — 

 remained largely as they now are throughout geologic time; 

 or rather have undergone extensive alterations, elevations 

 and depressions, denudations and rearrangements, as 

 claimed by Freeh, de Lapparent, Arldt and other recent 

 writers; whether the same geologic agencies as we now 

 observe were equally or more or less active In earlier 

 times? The remainder of this chapter will be devoted 

 to these and to related themes, while later chapters will 

 elucidate the subject further. 



I. The primitive plastic crust. 



No matter what view may be accepted as to the origin 

 and condition of the earth prior to the appearance of plant 

 and animal life on It, all present physical, geological and 

 palaeontologlcal facts Indicate that the surface crust, from 

 Archean to Permian time, was more plastic, uniformly 

 extended, and subject to variations in marine, freshwater, 

 and terrestrial conditions than during recent epochs. This 

 plastic nature of the crust prevented the formation of deep 

 depressions and equally of lofty mountain ranges that are 

 both Inimical to rapid organic evolution and activity. So, 

 even if It be granted that denudation has worn away the 



