BY THE TRESIDEXT. 183 



has yet been satisfactorily solved. A considerable quantity of 

 vegetable food is undoubtedly supplied from the Sargasso Sea and 

 a similar area in the Pacific Ocean, as well as by the seaweeds 

 which fringe every coast. But this supply is not sufficient for the 

 indirect support of the countless host of animals that inhabit the 

 depths of the ocean, all of which are necessarily zoophagous or 

 subsist on other animals. Plant-life, except, perhaps, a peculiar 

 kind, which will be presently noticed, appears to be absent in 

 depths exceeding 150 fathoms. 



But in all probability the chief supply of vegetable food is 

 derived from the countless diatoms, coccoliths, rhabdoliths, and 

 oscillatoriie, which are plants of a low degree of organization and 

 swarm on the surface of the sea ; these are swallowed by pelagic 

 animals (such as Salpm and Pteropoda, or "sea-butterflies"), and 

 the latter fall to the bottom after death, and form that floeculent 

 or glairy mass which I have described, in the Keport of the 

 ' Porcupine' Expedition of 1869, as covering the bed of the North 

 Atlantic at great depths. * The preservative effect of sea- water on 

 animal tissues would stay decomposition for a long while ; and Mr. 

 Moseley ascertained by a curious experiment that it would take only 

 about four days for a Salpa to reach the bottom at a depth of 2000 

 fathoms, and that the 8alpa was not greatly decomposed after 

 having remained in sea-water for a month in the tropics. 



"When we say that vegetable life does not exist at any con- 

 siderable depth, we must not forget that some kind is said to occur 

 in great abundance even in the benthal or deepest zone. The word 

 "benthal" is applied to depths exceeding 1000 fathoms (see my 

 Address which is referred to at p. 190 of this Lecture). Shells, 

 corals, and other organisms are everywhere permeated by what are 

 considered to be minute plants allied to fungi or confervse, which 

 form branching canals, like those of the Cliona or perforating 

 sponge ; and such canals have been also detected in all fossiliferous 

 strata of a marine nature, from the Silurian to the present epoch. 

 These plants, or Thallophytes, have been called "parasitic " ; but 

 they do not live on any other living thing. They can hardly serve 

 as food for deep-sea animals, because they are never exposed. 

 "Whether they may not be a link to connect the animal and 

 vegetable kingdoms may be a matter for further investigation. 



Food is of course a very important factor as regards the size of 

 all animals. 1 have noticed, in my work on 'British Conchology,' 

 that Mollusca from moderate depths are generally larger than those 

 of the same species from shallow water ; but this does not seem to 



* See 'Proc. Eoy. Soc' for 1870, p. 420. 



