INTRODUCTORY. 19 



of the water in constant movement, and produce 

 oscillations up and down, even in the lower ones, 

 by increasing or diminishing the column of water 

 resting on the bottom. The differences of tempera- 

 ture which are dependent on the alternation of day 

 and night, on changes of weather, and the course of 

 the seasons, cause expansions and displacements of 

 the constituents of the bottom. Into the greater 

 depths, where these forces can operate but rarely 

 or slightly, or even not at all, the currents of sinking 

 water, which has become heavier than the subjacent 

 strata, by cooling or increase of its amount of salt, 

 penetrate." Finally, he says, " Of all the movements 

 which convey organic materials to the sea- bottom, 

 descending currents are evidently among the most 

 efficacious. Their operation falls precisely in the 

 most suitable season for this purpose ; it commences 

 after the annual development of the marine vegeta- 

 tion in the temperate and cold zones has attained 

 its maximum, when strong and long-continued 

 storms gather their chief harvest in the fields of 

 Zostera (grass wrack) and tangle, and the bottom 

 of the sea is disquieted to a greater depth than 

 usual." 1 



This is, in effect, perhaps the most reasonable and 



1 " Whence comes the Nourishment for the Animals of the 

 Deep Seas," by Prof. Karl Mobius, in "Annals Nat. Hist.," 

 vol. viii. (1871), p. 193. 



C 2 



