200 Prof. K. MoLius on the Nourishment of 



The aquarium was now left entirely to itself. At the end 

 of four weeks the lower boundary of the slope had neverthe- 

 less advanced about 2 centims. further, and the horizontal 

 part of the bottom was covered with mud-particles still more 

 thickly tlian before. 



In both aquaria^ therefore, mechanical, thermic, and living 

 forces cooperated to bring about a forward movement of organic 

 materials from the higher towards the lower regions. 



Sand-grains and fragments of shells, when laid on the top, 

 pressed the organic mud-particles aside by sinking in betAveen 

 them. As gravity resists their ascent towards the shore, the 

 mass must on the whole go further downwards. 



When the bottom is heated in the higher regions, the vo- 

 lume of the constituents of its soil increases. In consequence 

 of this extension, the mass must move more downwards than 

 upwards, because gravity here also opposes movement up- 

 Avards. 



If a cooling of the water takes place above the shallower 

 regions, it becomes condensed, sinks down, and runs upon the 

 sloping ground down into the deeps, where there is warmer 

 and lighter water, which it displaces and replaces. The 

 bottom cuiTcnt carries light organic bodies with it into the 

 deeps. 



I'luctuations in the equilibrium of the water, and the rest- 

 lessness of animals which live on the bottom, both in the 

 higher and lower regions, their creeping about, tube-building, 

 seeking of nomishment, expulsion of indigestible materials, 

 respiration, and growth, keep the constituents of the super- 

 ficial layer of soil loose and in constant movement, so that 

 they can readily be carried away by the water flowing down- 

 wards. 



The same moving forces operate also in the sea. Here not 

 only is the extension of the water-basin infinitely greater, but 

 even the sum of the forces is enormously increased. 



Dead plants, fragments of shells, and sand are heaped one 

 upon the other to a height of feet or fathoms. The alterna- 

 tion of flood and ebb and the winds keep the upper strata of 

 the water in constant movomont, and produce oscillations up 

 and down, even in the lower ones, by increasing or diminish- 

 ing the column of water resting upon the bottom. The dif- 

 ferences of temperature which are dependent on tlie alterna- 

 tion of day and night, on changes of weather and the coiu-se 

 of the seasons, cause expansions and displacements of the 

 constituents of the bottom. Into the greater depths, where 

 these forces can operate but rarely and slightly, or even not 

 at all, the currents oi sinking \\ater, which has become heavier 



