224 THE ATLANTIC. (CHAP, I. 
ent the tow-net, which consists simply of a conical bag of mus- 
lin or bunting attached to an iron ring, is constantly open— 
descending, dragging along, and ascending. If worked on the 
surface, there is of course neither difficulty nor question ; but if 
it be brought up from 500 fathoms, at which depth it has been 
towing for some time, the net may be supposed to contain chief- 
ly the species living at that depth; but mixed with these there 
must be a considerable number of more superficial forms, some 
taken when the net was going down with its open mouth down- 
ward, and many more captured during its long ascent of half a 
mile through the upper layers. We can not, therefore, as yet 
say with certainty whether the surface species live in the deep- 
er belts or not, but we are justified in concluding that species 
which are absent on the surface, and present only when a cer- 
tain depth has been gained, are special to that and probably to 
greater depths. If, again, species differing both from those 
procured on the surface and at intermediate depths are found 
in the bottom deposits, it is a legitimate inference that these 
live below the zone of our deepest tow-net observations. 
Now, if it be the case that ooze-forming foraminifera are 
confined to an upper layer of say not more than 500 fathoms 
in thickness, the supply of their shells, and consequently the 
supply of the red clay, which, according to our view, is to a 
great extent the product of their decomposition, must be pretty 
constant over the area where foraminifera abound; while, on 
the other hand, if the Radiolarians live at all depths in the sea, 
the number of their skeletons falling to the bottom at one place 
must increase with the increasing depth of the water; and it be- 
comes quite intelligible that, in a bed which is being formed at 
the prodigious depth of five and a half nautical miles, the tests 
of the Radiolarians should so preponderate over the red clay 
as to entirely alter the character of the deposit. I must repeat, 
however, that it must not be supposed that these deep-sea for- 
mations which, from their general appearance, we put down on 
