XXll 
THE VOYAGE OF H.M.S. CHALLENGER. 
skeletons must, after the death of the organisms, form siliceous deposits of considerable 
extent around all coasts bordered with ice, at depths between 80 and 400 fathoms. 
Opposite Victoria Barrier the bottom w’as covered with a white or greenish mud, consist- 
ing principally of Diatom frustules. In very deep water, opposite Victoria and Graham’s 
Land, the mud was very pure and fine grained, but in shallow water, near the coast, it 
was mixed with sandy and gravelly particles. Hooker considered that these microscopic 
plants were intended to maintain in the south Polar regions the balance between the 
animal and vegetable kingdom, and also to purify the vitiated atmosphere, performing in 
Antarctic latitudes the part of vegetation in other regions. He states that Diatoms exist 
in every latitude from Spitzbergen to Victoria Land, Iceland, Great Britain, the Medi- 
terranean, North and South America, and the islands of the South Sea, and that the 
frustules of species living in the Antarctic have contributed to the formation of various 
strata during geological periods. He estimates that the deposit formed principally of 
Diatom frustules extends continuously for more than 400 miles off Victoria Land, at 
depths of about 300 fathoms. The existence of remains of Diatoms, including a few 
Antarctic species, in volcanic ashes, pumice, and scoriae, led him to suppose that organic 
substances covering the bases of active volcanoes, like Mount Erebus and Vesuvius, might 
be ejected from the craters along wfith volcanic products. 
In 1840 Edward Forbes joined, as naturalist, the surveying ship “ Beacon” while in 
the Mediterranean, and for eighteen months he studied the iEgean Sea and its shores, 
taking more than one hundred dredgings at different depths down to 130 fathoms. Before 
Forl)C8’ time the bathymetrical distribution of marine animals had been investigated to a 
certain extent, but the works of Audouin and Milne-Edwards (1830), Sars (1835), and 
Oersted (1844), ap[>lied only to the more superficial w^aters of the sea. Forbes studied 
the question with regard to animals inhabiting deep water, and in 1844 published his 
memoir, “ On the Light thrown on Geolog}'’ by Submarine Kesearches.” ^ He maintains 
that the dredgings show the extetence of distinct regions at successive depths, having 
each a special a.ssociation of species. He remarks that the' species found at the greatest 
depths are also found on the coasts of England, and he concludes, therefore, that such 
species have a wider geographical distribution. Forbes divided the area occupied by 
marine animals into eight zones of depth, in which animal life gradually diminished with 
increase of depth, until a zero was readied at about 300 fathoms. He shows that in 
Cretaceous and Tertiary layers similar zones may be distinguished, and that depth must 
have been in former times, as it is now, one of the factors in the distribution of marine 
organisms. He found fewer species in the deep zones than in the shallow ones, and 
supposes that j»lants, like animals, disapjieared at a certain depth, the zero of vegetable 
life being at a less depth than that of animal life. Forbes concluded that, as nearly all 
marine basins are over 300 fathoms in dejith, most of the sedimentary beds must be void 
' Edinhunjh New I'hil. Joum., vol. xxxvi. p. .318. 
