THE PELAGIC FAUNA AND FLOPIA. 



207 



The wide bathymetrical range of species belonging to the 

 principal groups of the animal kingdom shows that nearly all 

 littoral types can adapt themselves to the conditions of the deep 

 and there seems to be no reason why, in the oldest geolog- 



sea 



Fig-. lo2. — Pelagic Refuse. Magnified. 



ical periods, the same adaptation should not have taken place. 

 Yet, as we well know, no forms characteristic of the palaeozoic 

 period have been dredged from the abyssal regions of the sea. 

 We are therefore warranted in assuming that such a migration 

 did not take place from the littoral regions to the deep sea in 

 palaeozoic times, and that it was not before the end of the Juras- 

 sic or commencement of the cretaceous that the littoral fauna 

 began to find its way into deep water along the lines of the con- 

 tinental slopes. This colonization may have taken place, either 

 through the gradual migration of the adults from their shal- 

 lower habitat towards deeper regions, or, as I suggested in the 

 report upon the sea-urchins collected by Pourtales during the 



