76 Dr. John Murray [Feb. 28, 



may greatly exceed the number in polar waters, where, on the other 

 hand, there is often a great development of individuals, so that there 

 is probably a greater bulk of organic matter in the cold polar 

 waters than in the warm tropical waters. The rate of animal meta- 

 bolism is slower at a low than at a high temperature, and organisms 

 inhabiting tropical waters probably pass through their life-history 

 much more rapidly than similar organisms living in polar regions. 

 Carbonate-of-lime-secreting organisms are most abundant in the 

 warm tropical waters, decreasing in numbers towards the polar 

 regions, and it has been shown that the jDrecipitation of carbonate of 

 lime from solution in sea-water takes place much more rapidly at a 

 high temperature. The pelagic larvae of bottom-living species are 

 always present in the warm surface waters of the tropics, sometimes 

 growing to an enormous size ; but they are absent from the cold polar 

 waters and in the deep sea, where the majority of the bottom -living 

 species have a direct development. 



The Arctic fauna and flora, both at the surface and at the bottom, 

 resemble the Antarctic fauna and flora, and a large number of 

 identical and closely-related species are recorded from the two polar 

 areas, though quite unknown in the intervening tropical zone. 



The boundary line between the deep-sea region and the neritic 

 province is marked out by what has been called the " mud-line," 

 where the minute organic and inorganic particles derived from the 

 land and surface waters find a resting place upon the bottom, or 

 serve as food for enormous numbers of Crustacea, which in their turn 

 are the prey of fishes and the higher animals ; this mud-line, in fact, 

 appears to be the great feeding-ground in the ocean, and its average 

 depth is about 100 fatlioms along the borders of the great ocean basins. 



The majority of deep-sea species are mud eaters ; some are of 

 gigantic size ; some are armed with peculiar tactile, prehensile, and 

 alluring organs ; some are totally blind, whilst others have large eyes 

 and are provided with a kind of dark lantern for the emission of 

 phosphorescent light. The deep-sea fauna does not represent the 

 remnants of very ancient faunas, but has rather been the result of 

 migrations from the region of the mud-line in relatively recent 

 geological times. 



The Challenger investigations show that species are most abundant 

 in the shallow waters near land, decreasing in numbers with increasing 

 depth, and especially with increasing distance from continental land.* 

 This is true as a general rule, especially of tropical waters, but in 

 polar regions there are indications of a more abundant fauna in 

 depths of 50 to 150 fathoms than in shallower water under 50 

 fathoms. "I" 



* See ' Challenger Keports,' " A Summary of the Scientific Kesults," by 

 John Murray, pp. 1430-1436, 1895. 



t See Murray, " On the Deep and Shallow-Water Marine Fauna of the 

 Kergiielen Kegion of the Great Southern Ocean," Trans. Koy. Soc. Edin. 

 vol. xxxviii. p. 343, 1896. 



