Walther — Origin and Peopling of the Deep Sea. 63 



the more fundamental relationships have to consider it as Mes- 

 ozoic and would refer the majority of the forms to Cretaceous 

 and Jurassic genera and others to genera of the Triassic. The 

 few Paleozoic genera occurring at all water depths would 

 thereby gain no significance because all specific forms of Paleo- 

 zoic time are wanting in the deep sea, whereas, on the other 

 hand, many representatives of the same are found living in the 

 present shallow sea. 



General considerations as to the life habitats of the deep 

 sea have led us to the conviction that its fauna must have 

 migrated from the shallow sea ; the comparison of the deep sea 

 fauna with the fossil faunas has shown us that it has a Meso- 

 zoic character ; therefrom the conclusion necessarily follows 

 that the peopling of the deep sea can be traced at the earliest 

 to the Triassic. 



The deep sea basins represent the greatest inequalities of the 

 earth's crust. While the average height of the mainland 

 amounts to only 700 meters, the average depth of the oceans 

 is 3500 meters, but the average depth of the deep sea basins 

 amounts to about 5000 meters. It is only the Tibetan high- 

 land that rises to this amount above the mainland, while one- 

 half of the entire earth's surface is depressed below this level. 



In different periods of the earth's history great land areas 

 have sunk beneath the surface of the sea to be united to the 

 deep sea; into the new depressions the water flowed and left 

 the former shallow sea regions. Since the Jurassic, North 

 America and Europe have in this way gained land, and even 

 in Asia we see the land growing at the expense of the shallow 

 sea. The intensive development of mammals, birds and 

 insects since the Eocene seems to stand in the closest causal 

 connection thereto. 



The fact, recurring in all great sea basins, that the greatest 

 depths occur nearest the coasts, can be explained only by the 

 assumption that these deep channels represent local exagger- 

 ated deepenings of a general deepening process. 



From the study of newer and older mountains it is now seen 

 that hand in hand with the elevation of the mountain chains 

 extensive depressions have occurred. The Alpine folding con- 

 tinues into the depressed plain of Lombardy just as the chains 

 of the Himalayas are connected with the Bengal depression 

 and the South American Cordillera finds in the bottom of the 

 Pacific ocean its downward-directed compensation. With the 

 elevation of the Black Forest and Vosges the plain of the 

 Rhine valley sank into the depths, and the sinking of the Dead 

 Sea corresponds to the elevation of the Lebanon. 



We must also expect the beginnings of those immense 

 depression areas to be connected with the powerful processes 



