XII 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 343 
when first discovered, this fact constituting the test of the 
class to which an island belongs; whence he argued that none 
of them had ever been connected with continents, but all had 
originated in mid-ocean. These considerations alone render 
it almost certain that the areas now occupied by the great 
oceans have never, during known geological time, been 
occupied by continents, since it is in the highest degree im¬ 
probable that every fragment of those continents should have 
completely disappeared, and have been replaced by volcanic 
islands rising out of profound oceanic abysses ; but recent 
research into the depth of the oceans and the nature of the 
deposits now forming on their floors, adds greatly to the 
evidence in this direction, and renders it almost a certainty 
that they represent very ancient if not primaeval features of 
the earth’s surface. A very brief outline of the nature of this 
evidence will be now given. 
The researches of the Challenger expedition into the 
nature of the sea-bottom show, that the whole of the land 
debris brought down by rivers to the ocean (with the ex¬ 
ception of pumice and other floating matter), is deposited 
comparatively near to the shores, and that the fineness of the 
material is an indication of the distance to which it has been 
carried. Everything in the nature of gravel and sand is laid 
down within a very few miles of land, only the finer muddy 
sediments being carried out for 20 or 50 miles, and 
the very finest of all, under the most favourable conditions, 
rarely extending beyond 150, or at the utmost, 300 miles 
from land into the deep ocean. 1 Beyond these distances, and 
covering the entire ocean floor, are various oozes formed wholly 
from the debris of marine organisms ; while intermingled with 
these are found various volcanic products which have been 
either carried through the air or floated on the surface, and a 
small but perfectly recognisable quantity of meteoric matter. 
Ice-borne rocks are also found abundantly scattered over the 
ocean bottom witlyn a definite distance of the arctic and 
antarctic circles, clearly marking out the limit of floating ice¬ 
bergs in recent geological times. 
1 Even the extremely fine Mississippi mud. is nowhere found beyond a 
hundred miles from the mouths of the river in the Gulf of Mexico (A. Agassiz, 
Three Cruises of the Blake, vol. i. p. 128). 
