120 GEOGKAPHICAL DISTRIBUTION. 



of the sea. Such are the radiolarians, certain foraminifers and in- 

 fusorians, the siphonophorous medusae (Portuguese man-of-war, 

 Physalia; Velella, Porpita), winged-shells (pteropods), a limited 

 number of gasteropods (Atlanta, lanthina, Glaucus, &c.), cephalo- 

 pods, and tunicates (Salpa, Pyrosoma). The schizopod and ento- 

 mostracous Crustacea are numerously represented, while a genus of 

 hemipterous insect (Halobates) finds a suitable home clinging to the 

 waves at practically all distances from the land. A number of 

 fishes, such as the herrings, mackerel, tunny, swordfish, flying-fish 

 (Exoccetus), flying-gurnard (Dactyloptera), sea-horse, and most of 

 the sharks, some of which approach the shore during the spawning 

 season, might be added to this list, and among the Mammalia the 

 whales and dolphins. 



The primary condition governing the existence of a pelagic 

 fauna is manifestly the development over the oceanic expanse of 

 vegetable life. This is found, for the most part, in the microscopic 

 diatoms and the Oscillatoriae, the former of which abound more 

 particularly in high northern and southern latitudes, frequently, by 

 their vast numbers, rendering the water thick as soup, and impart- 

 ing to it a peculiar brownish or blackish tint, the so-called " black 

 water " of Arctic navigators. In the temperate and warmer seas 

 the diatoms are largely replaced by the oscillatorians, whose profuse 

 development is no less remarkable. In the Arafura Sea, between 

 Australia and New Guinea, the officers of the "Challenger" found 

 the water continuously discoloured during a period of several days' 

 sail, and giving out the odour of a reedy pond ; and in the Atlantic 

 they " passed for days through water full of minute algae (Tricho- 

 desmium), gleaming in the water like particles of mica." * It is to 

 a species of Trichodesmium (T. erythrasum) that is due the peculiar 

 red colouring frequently seen over stretches of the Red Sea. 



Were it not for this profuse vegetable growth the sea would 

 probably be, in great part, an uninhabited waste. The algae fur- 

 nish the necessary nutriment to the simpler forms of animal life, 

 which in turn yield their substances to those more highly organised. 

 In this manner a true interdependence of conditions, or balance of 

 life, is maintained. Professor Moseley, however, believes that in 

 some parts of the ocean the quantity of freely suspended vegetable 



* Moseley, " Nature," xxvi., p. 559. 



