THE PELAGIC FAUNA AND FLORA. 901 
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 
sea ; and there seems to be no reason why, in the oldest geolog- 
Fig. 132. — Pelagic Refuse. Magnified. 
ical periods, the same adaptation should not have taken place. 
Yet, as we well know, no forms characteristic of the paleozoic 
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 
palzeozoie 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 Pourtalés during the 
